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Convertible Senior Notes (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jul. 26, 2018
Aug. 31, 2017
Jun. 30, 2014
Debt Instrument [Line Items]          
Convertible senior notes, principal\ fair value amount $ 143,503 $ 140,974      
Convertible senior notes, debt issuance costs 788 1,182      
Convertible senior notes, net of debt issuance costs 142,715 139,792      
4.875% convertible senior notes (due 2020)          
Debt Instrument [Line Items]          
Convertible senior notes, principal\ fair value amount 113,000 113,000     $ 115,000
Convertible senior notes, debt issuance costs 788 1,182      
Convertible senior notes, net of debt issuance costs $ 112,212 111,818      
Debt instrument, interest rate 4.875%       4.875%
3.25% convertible senior notes (due 2020)          
Debt Instrument [Line Items]          
Convertible senior notes, principal\ fair value amount $ 30,503 27,974 $ 8,000    
Convertible senior notes, debt issuance costs 0 0      
Convertible senior notes, net of debt issuance costs $ 30,503 $ 27,974      
Debt instrument, interest rate 3.25%   3.25% 3.25%