Unsecured Financing Agreements (Details)		12 Months Ended
	Dec. 16, 2016 USD ($)	Dec. 31, 2016 USD ($) item $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Oct. 08, 2014 USD ($)	Jul. 03, 2013 USD ($)	Feb. 15, 2013 USD ($)
Convertible Senior Notes
Principal Amount		$ 2,053,229,000	$ 1,372,594,000
Conversion Spread Value - Shares | shares		2,697	97	3,432
Unamortized deferred financing costs		$ (45,732,000)	$ (38,336,000)
Carrying amount of debt components		2,011,544,000	1,323,795,000
Carrying amount of conversion option equity components recorded in additional paid-in capital		$ 45,988,000	$ 46,343,000
Closing share price (in dollars per share) | $ / shares		$ 21.95	$ 20.56	$ 23.24
Portion of repurchase price attributable to equity component recognized as a reduction of additional paid-in-capital		$ (355,000)	$ (17,727,000)	$ 15,568,000
Loss on extinguishment of debt		$ 8,781,000	$ 5,921,000
Conversion spread included in computation of diluted EPS (in shares) | shares		2,697,000	97,000	3,432,000
Interest Expense		$ 230,799,000	$ 202,550,000	$ 161,104,000
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
2017 Notes
Convertible Senior Notes
Conversion price (in dollars per share) | $ / shares		$ 23.96
Amount by which if-converted value of the Notes are less than principal amount		$ 34,600,000
Amount of debt repurchased		19,400,000
Amount paid for debt repurchased		19,900,000
Portion of repurchase price attributable to equity component recognized as a reduction of additional paid-in-capital		(400,000)
Loss on extinguishment of debt		$ 600,000
2018 Notes
Convertible Senior Notes
Conversion price (in dollars per share) | $ / shares		$ 21.15
Amount by which if-converted value of the Notes exceed principal amount		$ 22,700,000
2019 Notes
Convertible Senior Notes
Conversion price (in dollars per share) | $ / shares		$ 20.01
Amount by which if-converted value of the Notes exceed principal amount		$ 33,100,000
Amount of debt repurchased		19,400,000	118,600,000
Debt repurchased amount		$ 19,900,000	136,300,000
Amount paid for debt repurchased			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			5,900,000
Conversion spread included in computation of diluted EPS (in shares) | shares		2,700,000
2021 Senior Notes
Convertible Senior Notes
Amount issued	$ 700,000,000
Principal Amount		$ 700,000,000
Coupon Rate (as a percent)	5.00%	5.00%
Effective Rate (as a percent)		5.32%
Remaining Period of Amortization		5 years
Unamortized discount		$ (9,728,000)
2021 Senior Notes | Maximum
Convertible Senior Notes
Percentage of principal amount that may be redeemed	35.00%
2021 Senior Notes | Debt instrument redemption period one
Convertible Senior Notes
Percentage of principal amount that may be redeemed	100.00%
2021 Senior Notes | Debt instrument redemption period two
Convertible Senior Notes
Percentage of principal amount that may be redeemed	100.00%
Convertible Senior Notes
Convertible Senior Notes
Unamortized discount		(26,135,000)	(47,351,000)
Principal amount of notes, basis for conversion		$ 1,000
Amount of debt repurchased				0
Debt repurchased amount				0
Conversion of principal not included in computation of diluted EPS (in shares | shares		59,900,000
Minimum number of conditions to be satisfied for conversion of debt | item		1
Interest Expense		$ 57,100,000	58,000,000	$ 49,400,000
Convertible Senior Notes | Conversion upon satisfaction of closing market price condition
Convertible Senior Notes
Minimum trading period as a basis for debt conversion		20 days
Consecutive trading period as a basis for debt conversion		30 days
Convertible Senior Notes | 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%
Convertible Senior Notes | Conversion upon satisfaction of trading price condition
Convertible Senior Notes
Consecutive trading period as a basis for debt conversion		5 days
Convertible Senior Notes | 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%
Convertible Senior Notes | 2017 Notes
Convertible Senior Notes
Amount issued					$ 431,300,000
Principal Amount		$ 411,885,000	431,250,000
Coupon Rate (as a percent)		3.75%			3.75%
Effective Rate (as a percent)		5.86%
Remaining Period of Amortization		9 months 18 days
Conversion Rate		41.7397
Conversion price (in dollars per share) | $ / shares		$ 23.96
Amount by which if-converted value of the Notes are less than principal amount		$ 34,600,000
Conversion spread included in computation of diluted EPS (in shares) | shares		0
Convertible Senior Notes | 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%
Convertible Senior Notes | 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%
Effective Rate (as a percent)		6.10%
Remaining Period of Amortization		1 year 2 months 12 days
Conversion Rate		47.2712
Conversion price (in dollars per share) | $ / shares		$ 21.15
Conversion Spread Value - Shares | shares		1,097		1,221
Amount by which if-converted value of the Notes exceed principal amount		$ 22,700,000
Convertible Senior Notes | 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%
Effective Rate (as a percent)		5.35%
Remaining Period of Amortization		2 years
Conversion Rate		49.9717
Conversion price (in dollars per share) | $ / shares		$ 20.01
Conversion Spread Value - Shares | shares		1,600	97	2,211
Amount by which if-converted value of the Notes exceed principal amount		$ 33,100,000
Convertible Senior Notes | 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 Notes and Senior Notes
Convertible Senior Notes
Unamortized deferred financing costs		$ (5,822,000)	$ (1,448,000)