Debt (Details) (USD $)	1 Months Ended		3 Months Ended		1 Months Ended
	Feb. 28, 2015	Nov. 30, 2014	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015	Dec. 31, 2014	Oct. 31, 2011
Debt Conversion			$ 16,700,000
Debt Instrument, Face Amount							400,000,000
Repayments of Convertible Debt			16,686,000	0
Stock issued during period conversion of convertible notes			146,253
Gains (Losses) on Extinguishment of Debt			942,000	0
Treasury Stock, Shares			2,163,980			2,017,732
Treasury Stock, Value			181,817,000			169,530,000
Maximum reduction of warrants held by warrant holder		493,229	76,749
Limit price per share for warrant holders to close out hedge position		$ 397.75	$ 408.00
Payments for Repurchase of Warrants	124,000,000		124,531,000	0
Maximum amount of warrant payable			24,000,000
Credit Facility [Domain]
Line of Credit Facility, Maximum Borrowing Capacity			750,000,000
Line of Credit Facility, Expiration Period			5 years
Line of Credit Facility, Increase (Decrease), Net			250,000,000
Line of Credit Facility, Maximum Month-end Outstanding Amount			100,000,000
Line of Credit Facility, Fair Value of Amount Outstanding			0
Convertible senior notes
Treasury Stock, Shares			146,248
Treasury Stock, Value			12,300,000
Trading days to calculate settlement			40
Additional Paid-in Capital [Member]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			62,600,000
Adjustments to Additional Paid in Capital, Warrant Issued			23,300,000
Scenario, Forecast [Member]
Debt Conversion					127,300,000
Warrant Transactions [Member]
Accounts Payable, Other						$ 59,800,000
Reduction of number of warrants			416,480