Convertible Senior Notes - Narrative (Detail) (USD $)	12 Months Ended				1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Convertible Senior Notes 4.00 Percent Due 2015 [Member]	Dec. 31, 2010 Convertible Senior Notes 4.00 Percent Due 2015 [Member] Y	Dec. 31, 2011 Convertible Senior Notes 4.00 Percent Due 2015 [Member]	May 31, 2012 Convertible Senior Notes 5.00 Percent Due 2015 [Member] Y	Dec. 31, 2012 Convertible Senior Notes 5.00 Percent Due 2015 [Member]
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			2		2
Issued convertible senior notes, net proceeds following underwriting expenses			582,000,000		727,500,000
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		70.6980
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			$ 18.9400
Convertible senior notes convertible into common stock, subsequent conversion price		$ 14.1447
Intrinsic value of the contingent beneficial conversion feature		75,800,000		60,100,000
Convertible senior notes, unamortized discount		22,700,000		60,100,000		9,300,000
Repurchase of convertible senior notes		492,500,000
Net payment from extinguishment of 4% Convertible Senior Notes	617,476,000	617,476,000
Loss on extinguishment of 4% Convertible Senior Notes	(162,340,000)	162,340,000
Convertible senior notes, contingent beneficial conversion feature	$ 11,717,000				$ 11,700