Convertible Senior Notes - Narrative (Detail) (USD $)	12 Months Ended				1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2013 Convertible Senior Notes 4.00 Percent Due 2015	Dec. 31, 2010 Convertible Senior Notes 4.00 Percent Due 2015	Dec. 31, 2012 Convertible Senior Notes 4.00 Percent Due 2015	May 31, 2012 Convertible Senior Notes 5.00 Percent Due 2015	Dec. 31, 2013 Convertible Senior Notes 5.00 Percent Due 2015	Dec. 31, 2012 Convertible Senior Notes 5.00 Percent Due 2015
Convertible Senior Notes:
Issued convertible senior notes, aggregate principal amount			$ 600,000,000		$ 750,000,000
Senior debt maturity date			Feb. 15, 2015		May 15, 2015
Issued convertible senior notes, interest rate			4.00%		5.00%
Issued convertible senior notes, frequency of interest payments per year		Semi-annually				Semi-annually
Issued convertible senior notes, net proceeds following underwriting expenses	727,500,000		582,000,000		727,500,000
Repurchase of Convertible senior notes		492,500,000
Convertible senior notes convertible into common stock, initial conversion rate, number of shares			46.6070			52.7969
Convertible senior notes convertible into common stock, subsequent conversion rate, number of shares		79.7379
Convertible senior notes, principal amount applicable for conversion rate			1,000		1,000
Convertible senior notes convertible into common stock, initial conversion price			$ 21.4560			$ 18.94
Convertible senior notes convertible into common stock, subsequent conversion price		$ 12.5411
Intrinsic value of the contingent beneficial conversion feature		93,200,000		75,800,000
Convertible senior notes, unamortized discount		26,900,000		22,700,000		5,400,000	9,300,000
Convertible senior notes, contingent beneficial conversion feature	$ 11,717,000				$ 11,700,000