Common Stock - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Aug. 31, 2015	May. 31, 2013	Oct. 31, 2012	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
Overview Of The Company [Line Items]
Common stock shares sold	3,099,173	3,902,862	7,964,601	131,425,000		125,688,000
Proceeds from issuance of common stock in public offering	$ 738,300	$ 355,100	$ 222,100	$ 730,000	$ 360,000
1.50% Convertible Senior Notes [Member]
Overview Of The Company [Line Items]
Principal amount of convertible senior notes		$ 660,000
Debt instrument interest rate		1.50%
Proceeds from convertible senior notes, net of underwriting discounts and offering costs		$ 648,000
Chief Executive Officer [Member]
Overview Of The Company [Line Items]
Common stock shares sold	82,645	487,857	35,398
Proceeds from issuance of common stock in public offering	$ 20,000	$ 45,000	$ 1,000
Chief Executive Officer [Member] | Private placement [Member]
Overview Of The Company [Line Items]
Common stock shares sold		596,272
Proceeds from issuance of common stock in public offering		$ 55,000