Convertible Senior Notes - Narrative (Detail) (USD $)	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2010	May 31, 2012	May 31, 2012	Nov. 30, 2014	Dec. 31, 2013
Convertible Senior Notes:
Issued convertible senior notes, aggregate principal amount		$ 146,553,000					$ 19,332,000
Issued convertible senior notes, interest rate						4.03%
Issued convertible senior notes, net proceeds following underwriting expenses	727,500,000
Convertible senior notes, contingent beneficial conversion feature	11,717,000
Convertible Senior Notes 4.00 Percent Due 2015
Convertible Senior Notes:
Issued convertible senior notes, aggregate principal amount			600,000,000
Senior debt maturity date			Feb. 15, 2015
Issued convertible senior notes, interest rate			4.00%
Issued convertible senior notes, frequency of interest payments per year		Semi-annually
Issued convertible senior notes, net proceeds following underwriting expenses			582,000,000
Repurchase of Convertible senior notes		492,500,000
Convertible senior notes convertible into common stock, initial conversion rate, number of shares			46.6070
Convertible senior notes convertible into common stock, subsequent conversion rate, number of shares		88.7389
Convertible senior notes, principal amount applicable for conversion rate			1,000
Convertible senior notes convertible into common stock, initial conversion price			$ 21.4560
Convertible senior notes convertible into common stock, subsequent conversion price		$ 11.2690
Intrinsic value of the contingent beneficial conversion feature		110,500,000					93,200,000
Convertible senior notes, unamortized discount		10,800,000					26,900,000
Convertible Senior Notes 5.00 Percent Due 2015
Convertible Senior Notes:
Issued convertible senior notes, aggregate principal amount				750,000,000	750,000,000
Senior debt maturity date					May 15, 2015
Issued convertible senior notes, interest rate				5.00%	5.00%
Issued convertible senior notes, frequency of interest payments per year		Semi-annually
Issued convertible senior notes, net proceeds following underwriting expenses					727,500,000
Convertible senior notes convertible into common stock, initial conversion rate, number of shares		52.7969
Convertible senior notes, principal amount applicable for conversion rate					1,000
Convertible senior notes convertible into common stock, initial conversion price		$ 18.94
Convertible senior notes, unamortized discount		1,500,000					5,400,000
Convertible senior notes, contingent beneficial conversion feature				$ 11,700,000