Convertible Senior Notes - Narrative (Detail) (USD $)	3 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended
	Mar. 31, 2014 Convertible Senior Notes 4.00 Percent Due 2015	Dec. 31, 2010 Convertible Senior Notes 4.00 Percent Due 2015	Dec. 31, 2013 Convertible Senior Notes 4.00 Percent Due 2015	May 31, 2012 Convertible Senior Notes 5.00 Percent Due 2015	Mar. 31, 2014 Convertible Senior Notes 5.00 Percent Due 2015	Dec. 31, 2013 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		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	81.9145
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	$ 12.2079
Intrinsic value of the contingent beneficial conversion feature	97,400,000		93,200,000
Convertible senior notes, unamortized discount	25,700,000		26,900,000		4,400,000	5,400,000
Convertible senior notes, contingent beneficial conversion feature				$ 11,700,000