Convertible Senior Notes (Details) (USD $)	3 Months Ended	6 Months Ended							3 Months Ended	6 Months Ended	12 Months Ended	2 Months Ended	6 Months Ended			2 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Conversion upon satisfaction of closing market price condition D item	Jun. 30, 2014 Conversion upon satisfaction of closing market price condition Minimum	Jun. 30, 2014 Conversion upon satisfaction of closing market price condition Maximum	Jun. 30, 2014 Conversion upon satisfaction of trading price condition	Jun. 30, 2014 Conversion upon satisfaction of trading price condition Maximum	Jun. 30, 2014 Convertible Senior Notes	Jun. 30, 2014 Convertible Senior Notes	Dec. 31, 2013 Convertible Senior Notes	Feb. 03, 2014 2018 Notes	Jun. 30, 2014 2018 Notes	Jan. 31, 2014 2018 Notes	Feb. 15, 2013 2018 Notes	Feb. 03, 2014 2019 Notes	Jun. 30, 2014 2019 Notes	Jan. 31, 2014 2019 Notes	Jul. 03, 2013 2019 Notes
Convertible Senior Notes
Amount issued															$ 600,000,000				$ 460,000,000
Interest rate (as a percent)													4.55%				4.00%
Principal Amount									1,059,978,000	1,059,978,000	1,060,000,000		599,981,000				459,997,000
Effective Rate (as a percent)													6.08%				5.37%
Conversion Rate													44.6455				47.5456
Remaining Period of Amortization													3 years 8 months 12 days				4 years 6 months
Net unamortized discount									(56,131,000)	(56,131,000)	(62,149,000)
Total									1,003,847,000	1,003,847,000	997,851,000
Carrying amount of conversion option equity components recorded in additional paid-in capital			28,107,000							48,502,000	48,502,000
Principal amount of notes, basis for conversion									1,000	1,000
Amount by which if-converted value of the Notes exceed principal amount													36,700,000				59,900,000
Closing share price (in dollars per share)													$ 23.77				$ 23.77
Conversion price (in dollars per share)													$ 22.40				$ 21.03
Number of anti-dilutive common shares excluded from the calculation of diluted income per share									44,600,000	44,800,000
Conversion spread value included in computation of diluted EPS	4,056,000	3,906,000							4,100,000	3,900,000
Deferred financing costs, net of amortization									1,500,000	1,500,000	1,600,000
Minimum number of conditions to be satisfied for conversion of debt				1
Percentage of conversion price as a basis for debt conversion					130.00%
Minimum trading period as a basis for debt conversion				20
Consecutive trading period as a basis for debt conversion				30 days			5 days
Percentage of conversion price and last reported sales price as a basis for debt conversion								98.00%
Period of average closing market price of common stock as a basis for debt conversion						10 days
Percentage of per share value of distributions that exceeds the market price of the entity's common stock as a basis for debt conversion					10.00%
Notice of conversion, principal amount												$ 19,000				$ 3,000
Quarterly dividend threshold amount per share of common stock												$ 0.3548		$ 0.44		$ 0.3710		$ 0.46