Debt - Summary (Details) - USD ($) |
Sep. 30, 2019 |
Dec. 31, 2018 |
Dec. 31, 2014 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Revolving credit facility balance | $ 100,000,000 | $ 0 | |
| Convertible Notes due 2019 | |||
| Debt Instrument [Line Items] | |||
| Face amount | 172,500,000 | 172,500,000 | $ 172,500,000 |
| Unaccreted discount on Convertible Notes | (1,338,000) | (5,890,000) | |
| Unamortized issuance costs on Convertible Notes | (196,000) | (899,000) | |
| Convertible Notes carrying value | 170,966,000 | 165,711,000 | |
| Convertible Notes due 2023 | |||
| Debt Instrument [Line Items] | |||
| Face amount | 345,000,000 | 345,000,000 | |
| Unaccreted discount on Convertible Notes | (35,805,000) | (42,641,000) | |
| Unamortized issuance costs on Convertible Notes | (6,410,000) | (7,634,000) | |
| Convertible Notes carrying value | $ 302,785,000 | $ 294,725,000 |
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- Definition Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Face (par) amount of debt instrument at time of issuance. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available.
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- Definition Amount, after accumulated amortization, of debt discount. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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