Convertible Senior Notes (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2014	Jul. 31, 2013
Debt Instrument [Line Items]
Convertible senior notes, principal\ fair value amount	$ 157,668	$ 156,647
Convertible senior notes, debt issuance costs	1,833	2,075
Convertible senior notes, net of debt issuance costs	155,835	154,572
4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Convertible senior notes, principal\ fair value amount	21,178	21,178		$ 100,000
Convertible senior notes, debt issuance costs	59	103
Convertible senior notes, net of debt issuance costs	21,119	21,075
3.25% Convertible Senior Notes (due 2020)
Debt Instrument [Line Items]
Convertible senior notes, principal\ fair value amount	23,490	22,469
Convertible senior notes, net of debt issuance costs	23,490	22,469
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	1,774	1,972
Convertible senior notes, net of debt issuance costs	$ 111,226	$ 111,028