Long-term Debt (Details) (USD $)	3 Months Ended			3 Months Ended			1 Months Ended		1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Senior Convertible Note [Member]	Mar. 31, 2013 Senior Convertible Note [Member] Minimum [Member]	Mar. 31, 2013 Senior Convertible Note [Member] Maximum [Member]	Feb. 28, 2013 5.375% Senior Notes [Member]	Mar. 31, 2013 5.375% Senior Notes [Member]	Feb. 28, 2013 8.50% Senior Notes [Member]	Apr. 26, 2013 Subsequent Event [Member]	Apr. 26, 2013 Subsequent Event [Member] Senior Convertible Note [Member]
Debt Instrument [Line Items]
Long-term debt outstanding	$ 500,000,000		$ 200,000,000	$ 200,000,000					$ 200,000,000		$ 200,000,000
Interest rate				0.00%			5.375%	5.375%	8.50%
Senior notes, maturity date				Dec. 01, 2018
Earliest conversion date				May 28, 2012
Weighted average price of common stock				$ 111.54
Trading days prior to conversion date required for conversion					50	65
Common stock, shares issued	56,143,986		55,587,167							2,300,000
Common stock conversion ratio										11.6553
Fair value at conversion										$ 216.99
Face amount							500,000,000
Redemption prices, percent of outstanding principal								101.00%
Proceeds from issuance of long-term debt							490,600,000
Debt issuance cost							9,400,000
Repayments							224,500,000
Make-whole premium									19,400,000
Interest payable									5,100,000
Loss on extinguishment of debt	25,129,000	0							25,100,000
Write off of unamortized deferred financing fees									4,200,000
Senior notes, fair value								$ 496,300,000