Long-term Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended	12 Months Ended
	Dec. 17, 2012	Mar. 31, 2013	Dec. 31, 2012
Debt instrument
Interest rate (as a percent)			2.625%
Carrying amounts of the liability components
Carrying amount of the liability components		$ 1,002,055,000	$ 991,191,000
Notes
Debt instrument
Aggregate principal amount of notes issued	1,380,000,000
Interest rate (as a percent)	2.625%	2.625%	2.625%
Initial conversion rate of common stock	28.023
Initial conversion price per share of common stock (in dollars per share)	$ 35.68
Number of underlying shares that the holder of the debt instrument would receive upon conversion	38.7
Repurchase price as a percentage of principal amount of debt instrument	100.00%
Specified minimum percentage of principal amount, the holders of which may declare all principal, accrued and unpaid interest to be due and payable immediately, upon the occurrence of an Event of Default	25.00%
Percentage of principal amount, which may be declared by holders of a specified principal amount to be due and payable immediately upon occurrence of an Event of Default	100.00%
Fair value of the notes excluding conversion feature at the date of issuance	989,500,000
Effective interest rate used to amortize liability component of debt issue costs (as a percent)	8.40%
Debt issue cost	32,200,000
Debt issue cost allocated to the liability component	23,100,000
Debt issue cost allocated to the equity component	9,100,000	9,124,000	9,124,000
Carrying amounts of the liability components
Principal Amount		1,380,000,000	1,380,000,000
Unamortized discount		(377,945,000)	(388,809,000)
Carrying amount of the liability components		1,002,055,000	991,191,000
Remaining term of debt		7 years
Carrying amounts of the equity components
Debt discount relating to value of conversion option		390,540,000	390,540,000
Debt issue costs	(9,100,000)	(9,124,000)	(9,124,000)
Carrying amount of the equity components		381,416,000	381,416,000
Fair value of the notes excluding conversion feature		$ 974,600,000	$ 989,500,000