Convertible Senior Notes - Key Component of Convertible Senior Notes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Contractual interest expense	$ 373	$ 2,184	$ 4,191
Amortization of debt issuance costs recognized as interest expense	103	844	1,172
Interest Expense, Debt, Total	476	3,028	5,363
3.25% Convertible senior notes due in 2020
Debt Instrument [Line Items]
Contractual interest expense	815	103	0
Amortization of debt issuance costs recognized as interest expense	0	0	0
Interest Expense, Debt, Total	815	103	0
4.875% convertible senior notes (due 2020)
Debt Instrument [Line Items]
Contractual interest expense	5,509	5,509	5,508
Amortization of debt issuance costs recognized as interest expense	789	789	804
Interest Expense, Debt, Total	$ 6,298	$ 6,298	$ 6,312