Convertible Senior Notes (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2014	Jul. 31, 2013
Debt Instrument [Line Items]
Convertible senior notes, principal amount	$ 155,728	$ 206,865
Convertible senior notes, debt issuance costs	2,753	3,858
Convertible senior notes, net of debt issuance costs	152,975	203,007
4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Convertible senior notes, principal amount	42,728	93,865		$ 100,000
Convertible senior notes, debt issuance costs	387	1,098
Convertible senior notes, net of debt issuance costs	42,341	92,767
4.875% Convertible Senior Notes (due 2020)
Debt Instrument [Line Items]
Convertible senior notes, principal amount	113,000	113,000	$ 115,000
Convertible senior notes, debt issuance costs	2,366	2,760
Convertible senior notes, net of debt issuance costs	$ 110,634	$ 110,240