Convertible Senior Notes (Details) $ / shares in Units, shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) shares	Jun. 30, 2016 USD ($) item $ / shares shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2015 USD ($)	Oct. 08, 2014 USD ($)	Jul. 03, 2013 USD ($)	Feb. 15, 2013 USD ($)
Convertible Senior Notes
Interest expense	$ 57,635,000	$ 49,799,000	$ 114,155,000	$ 100,333,000
Unamortized deferred financing costs	$ (32,588,000)		$ (32,588,000)		$ (38,336,000)
Portion of repurchase price attributable to equity component recognized as a reduction of additional paid-in-capital				(17,727,000)
Loss on extinguishment of debt		$ 629,000		$ 5,921,000
Conversion spread included in computation of diluted EPS (in shares) | shares	441	649	456	644
Deferred financing costs, net of amortization	$ 32,588,000		$ 32,588,000		38,336,000
Conversion upon satisfaction of closing market price condition
Convertible Senior Notes
Minimum number of conditions to be satisfied for conversion of debt | item			1
Minimum trading period as a basis for debt conversion			20 days
Consecutive trading period as a basis for debt conversion			30 days
Conversion upon satisfaction of closing market price condition | Minimum
Convertible Senior Notes
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%
Conversion upon satisfaction of closing market price condition | Maximum
Convertible Senior Notes
Period of average closing market price of common stock as a basis for debt conversion			10 days
Conversion upon satisfaction of trading price condition
Convertible Senior Notes
Consecutive trading period as a basis for debt conversion			5 days
Conversion upon satisfaction of trading price condition | Maximum
Convertible Senior Notes
Percentage of conversion price and last reported sales price as a basis for debt conversion			98.00%
2017 Notes
Convertible Senior Notes
Amount issued						$ 431,300,000
Principal Amount	$ 431,250,000		$ 431,250,000
Coupon Rate (as a percent)	3.75%		3.75%			3.75%
Effective Rate (as a percent)	5.87%		5.87%
Conversion Rate			41.7397
Remaining Period of Amortization			1 year 3 months 18 days
Amount by which if-converted value of the Notes are less than principal amount			$ 58,300,000
Conversion price (in dollars per share) | $ / shares	$ 23.96		$ 23.96
Conversion spread included in computation of diluted EPS (in shares) | shares	0		0
2017 Notes | Conversion upon satisfaction of closing market price condition | Minimum
Convertible Senior Notes
Percentage of conversion price as a basis for debt conversion			110.00%
2018 Notes
Convertible Senior Notes
Amount issued								$ 600,000,000
Principal Amount	$ 599,981,000		$ 599,981,000
Coupon Rate (as a percent)	4.55%		4.55%					4.55%
Effective Rate (as a percent)	6.10%		6.10%
Conversion Rate			46.7513
Remaining Period of Amortization			1 year 8 months 12 days
Amount by which if-converted value of the Notes are less than principal amount			$ 18,800,000
Conversion price (in dollars per share) | $ / shares	$ 21.39		$ 21.39
Conversion spread included in computation of diluted EPS (in shares) | shares	0		0
2019 Notes
Convertible Senior Notes
Amount issued							$ 460,000,000
Principal Amount	$ 341,363,000		$ 341,363,000
Coupon Rate (as a percent)	4.00%	4.00%	4.00%	4.00%			4.00%
Effective Rate (as a percent)	5.35%		5.35%
Conversion Rate			49.4927
Remaining Period of Amortization			2 years 6 months
Amount by which if-converted value of the Notes are less than principal amount			$ 8,800,000
Closing share price (in dollars per share) | $ / shares	$ 20.72		$ 20.72
Conversion price (in dollars per share) | $ / shares	$ 20.20		$ 20.20
Aggregate principal amount of debt repurchased		$ 14,500,000		$ 118,600,000
Amount paid for debt repurchased		16,500,000		136,300,000
Portion of repurchase price attributable to equity component recognized as a reduction of additional paid-in-capital				17,700,000
Repurchase expenses				100,000
Loss on extinguishment of debt		$ 600,000		$ 5,900,000
Conversion spread included in computation of diluted EPS (in shares) | shares	400		500
2018 Notes and 2019 Notes | Conversion upon satisfaction of closing market price condition | Minimum
Convertible Senior Notes
Percentage of conversion price as a basis for debt conversion			130.00%
Convertible Senior Notes
Convertible Senior Notes
Principal Amount	$ 1,372,594,000		$ 1,372,594,000		1,372,594,000
Unamortized discount	(37,055,000)		(37,055,000)		(47,351,000)
Unamortized deferred financing costs	(1,115,000)		(1,115,000)		(1,448,000)
Carrying amount of debt components	1,334,424,000		1,334,424,000		1,323,795,000
Carrying amount of conversion option equity components recorded in additional paid-in capital	$ 46,343,000		$ 46,343,000		46,343,000
Principal amount of notes, basis for conversion					1,000
Closing share price (in dollars per share) | $ / shares	$ 20.72		$ 20.72
Conversion of principal not included in computation of diluted EPS (in shares | shares			62,500
Deferred financing costs, net of amortization	$ 1,115,000		$ 1,115,000		$ 1,448,000
Convertible Senior Notes | 2019 Notes
Convertible Senior Notes
Amount by which if-converted value of the Notes exceed principal amount			$ 8,800,000