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Long-term debt (Details)
$ / shares in Units, $ in Thousands
1 Months Ended 9 Months Ended 12 Months Ended
Aug. 12, 2016
shares
$ / shares
Dec. 11, 2013
USD ($)
Unit
Jan. 29, 2014
USD ($)
shares
$ / shares
Sep. 30, 2014
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Long-Term Debt [Line Items]              
Proceeds from borrowings on long-term indebtedness         $ 0 $ 2,000 $ 1,000
Payments on borrowings on long-term indebtedness         $ 0 0 $ 62,000
BAML Revolving Credit Facility [Memeber]              
Long-Term Debt [Line Items]              
Maturity date   Dec. 11, 2018          
Maximum borrowing capacity   $ 100,000          
Credit Agreement [Member]              
Long-Term Debt [Line Items]              
Number of lending institutions | Unit   3          
Debt covenant, consolidated debt service coverage ratio, minimum         2.50    
Debt covenant, leverage ratio, maximum       4.00 3.50   3.75
Debt covenant, minimum cash and liquid investments balance         $ 50,000    
2.875% Convertible Senior Notes Due 2021 [Member]              
Long-Term Debt [Line Items]              
Face amount of debt instrument     $ 250,000        
Interest rate percentage     2.875%        
Maturity date     Jan. 15, 2021        
Conversion rate of notes per $1,000 principal amount (in shares) | shares 32.3860   30.8821        
Conversion price per share (in dollars per share) | $ / shares $ 30.88   $ 32.38        
Debt issuance costs     $ 8,300        
BAML Term Loan [Member]              
Long-Term Debt [Line Items]              
Debt issuance costs             $ 1,800
Amount of debt issuance costs in other assets, netted against long term debt [Member]              
Long-Term Debt [Line Items]              
Debt issuance costs           4,900  
Amount of debt issuance costs in prepaid and other current assets, netted against long term debt [Member]              
Long-Term Debt [Line Items]              
Debt issuance costs           $ 1,200