Unsecured Secured Notes (Details) $ / shares in Units, shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Unsecured Senior Notes
Carrying amount of debt components	$ 1,924,711,000		$ 1,924,711,000		$ 1,998,831,000
Interest expense	130,126,000	$ 91,592,000	264,798,000	$ 178,775,000
Loss on extinguishment of debt	2,816,000	$ 186,000	6,114,000	$ 186,000
Conversion Spread Value - Shares | shares		1,863		1,900
Principal amount of notes, basis for conversion	$ 1,000		$ 1,000
Closing share price (in dollars per share) | $ / shares	$ 22.72	$ 21.71	$ 22.72	$ 21.71
2019 Notes
Unsecured Senior Notes
Principal Amount					77,969,000
Shares issued to settle redemption | shares			3,600
Value of shares issued to settle redemption			$ 78,000,000.0
Conversion Spread Value - Shares | shares		1,863		1,900
2021 Senior Notes 3.625%
Unsecured Senior Notes
Coupon Rate (as a percent)	3.63%		3.63%
Effective Rate (as a percent)	3.89%		3.89%
Remaining Period of Amortization			1 year 7 months 6 days
Principal Amount	$ 500,000,000		$ 500,000,000		500,000,000
2021 Senior Notes 5.00%
Unsecured Senior Notes
Coupon Rate (as a percent)	5.00%		5.00%
Effective Rate (as a percent)	5.32%		5.32%
Remaining Period of Amortization			2 years 6 months
Principal Amount	$ 700,000,000		$ 700,000,000		700,000,000
2023 Notes
Unsecured Senior Notes
Coupon Rate (as a percent)	4.38%		4.38%
Effective Rate (as a percent)	4.86%		4.86%
Remaining Period of Amortization			3 years 9 months 18 days
Principal Amount	$ 250,000,000		$ 250,000,000		250,000,000
Conversion Rate			38.5959
Conversion price (in dollars per share) | $ / shares	$ 25.91		$ 25.91
Closing share price (in dollars per share) | $ / shares	$ 22.72		$ 22.72
If-converted value	$ 219,200,000		$ 219,200,000
Amount by which if-converted value of the Notes are less than principal amount			$ 30,800,000
2025 Senior Notes
Unsecured Senior Notes
Coupon Rate (as a percent)	4.75%		4.75%
Effective Rate (as a percent)	5.04%		5.04%
Remaining Period of Amortization			5 years 8 months 12 days
Principal Amount	$ 500,000,000		$ 500,000,000		500,000,000
Unsecured Senior Notes
Unsecured Senior Notes
Principal Amount	1,950,000,000		1,950,000,000		2,027,969,000
Unamortized deferred financing costs	(6,870,000)		(6,870,000)		(8,078,000)
Carrying amount of conversion option equity components recorded in additional paid-in capital for outstanding convertible notes	3,755,000		3,755,000		3,755,000
Carrying amount of debt components	1,924,711,000		1,924,711,000		1,998,831,000
Convertible Senior Notes
Unsecured Senior Notes
Unamortized discount	$ (4,118,000)		$ (4,118,000)		(4,644,000)
Convertible Senior Notes | 2019 Notes
Unsecured Senior Notes
Coupon Rate (as a percent)	4.00%		4.00%
Interest expense	$ 3,000,000.0	$ 7,600,000	$ 6,200,000	$ 18,900,000
Shares issued to settle redemption | shares			3,600
Cash payments to settle redemptions			$ 12,000,000.0
Value of shares issued to settle redemption			78,000,000.0
Senior Notes
Unsecured Senior Notes
Unamortized discount	$ (14,301,000)		$ (14,301,000)		$ (16,416,000)