Convertible Senior Notes - Narrative (Detail) (USD $)	3 Months Ended	9 Months Ended	3 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 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	Sep. 30, 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		727,500,000		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			68.5105
Convertible senior notes, principal amount applicable for conversion rate				1,000		1,000
Convertible senior notes convertible into common stock, initial conversion price				$ 21.456			$ 18.94
Convertible senior notes convertible into common stock, subsequent conversion price			$ 14.5963
Intrinsic value of the contingent beneficial conversion feature			80,900,000		67,600,000
Convertible senior notes, unamortized discount			59,300,000		60,100,000		10,300,000
Repurchase of convertible senior notes			280,700,000
Net payment from extinguishment of 4% Convertible Senior Notes	357,220,000	357,220,000	357,220,000
Loss on extinguishment of 4% Convertible Senior Notes	(87,328,000)	(87,328,000)	87,328,000
Convertible senior notes, contingent beneficial conversion feature	$ 11,717,000	$ 11,717,000				$ 11,717,000