Annual report pursuant to Section 13 and 15(d)

Debt (Credit Agreement) (Details)

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Debt (Credit Agreement) (Details) - USD ($)
12 Months Ended
Nov. 19, 2015
Dec. 31, 2018
Dec. 31, 2017
Jul. 18, 2017
Dec. 31, 2014
Debt          
Revolving credit facility   $ 0 $ 81,168,000    
Credit Agreement          
Debt          
Credit facility amount         $ 70,000,000
Voting equity of foreign subsidiary pledged (as a percent) 66.00%        
Non-voting equity of foreign subsidiary pledged (as a percent) 100.00%        
Commitment fee   0.25%      
Revolving credit facility   $ 0 $ 81,168,000    
Credit Agreement | Other non-current assets          
Debt          
Debt issuance cost, outstanding   2,251,000      
Letters of credit          
Debt          
Credit facility amount       $ 5,000,000 $ 5,000,000
Second Amended and Restated Credit Agreement          
Debt          
Credit facility amount       350,000,000  
Right to increase credit facility, amount       $ 50,000,000  
Second Amended and Restated Credit Agreement | Prepaid expense and other current assets          
Debt          
Debt issuance cost, outstanding   $ 855,000      
Minimum | Credit Agreement | LIBOR          
Debt          
Spread on variable rate basis (as a percent) 1.50%        
Maximum | Credit Agreement | LIBOR          
Debt          
Spread on variable rate basis (as a percent) 3.25%