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Key Component of Convertible Senior Notes (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
4.25% Convertible Senior Notes (due 2018)      
Debt Instrument [Line Items]      
Contractual interest expense $ 2,184 $ 4,191 $ 4,250
Amortization of debt discount and debt issuance costs recognized as interest expense 844 1,172 836
Interest Expense, Debt, Total 3,028 5,363 5,086
3.25% Convertible Senior Notes (due 2020)      
Debt Instrument [Line Items]      
Contractual interest expense 103    
Interest Expense, Debt, Total 103    
4.875% Convertible Senior Notes (due 2020)      
Debt Instrument [Line Items]      
Contractual interest expense 5,509 5,508 5,606
Amortization of debt discount and debt issuance costs recognized as interest expense 789 804 811
Interest Expense, Debt, Total $ 6,298 $ 6,312 $ 6,417