Unsecured Senior Notes (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2019 USD ($)
Unsecured Senior Notes
Interest expense	$ 120,025,000	$ 134,672,000
Principal amount of notes, basis for conversion	$ 1,000
Closing share price (in dollars per share) | $ / shares	$ 10.25	$ 22.35
2021 Senior Notes 3.625%
Unsecured Senior Notes
Coupon Rate (as a percent)	3.63%
Effective Rate (as a percent)	3.89%
Remaining Period of Amortization	9 months 18 days
Principal Amount	$ 500,000,000		$ 500,000,000
2021 Senior Notes 5.00%
Unsecured Senior Notes
Coupon Rate (as a percent)	5.00%
Effective Rate (as a percent)	5.32%
Remaining Period of Amortization	1 year 8 months 12 days
Principal Amount	$ 700,000,000		700,000,000
2023 Notes
Unsecured Senior Notes
Coupon Rate (as a percent)	4.38%
Effective Rate (as a percent)	4.86%
Remaining Period of Amortization	3 years
Principal Amount	$ 250,000,000		250,000,000
Conversion Rate	38.5959
Conversion price (in dollars per share) | $ / shares	$ 25.91
Closing share price (in dollars per share) | $ / shares	$ 10.25
If-converted value	$ 98,900,000
Amount by which if-converted value of the Notes are less than principal amount	$ 151,100,000
2025 Senior Notes
Unsecured Senior Notes
Coupon Rate (as a percent)	4.75%
Effective Rate (as a percent)	5.04%
Remaining Period of Amortization	5 years
Principal Amount	$ 500,000,000		500,000,000
Unsecured Senior Notes
Unsecured Senior Notes
Principal Amount	1,950,000,000		1,950,000,000
Unamortized deferred financing costs	(5,026,000)		(5,624,000)
Carrying amount of conversion option equity components recorded in additional paid-in capital for outstanding convertible notes	3,755,000		3,755,000
Carrying amount of debt components	1,930,584,000		1,928,622,000
Convertible Senior Notes
Unsecured Senior Notes
Unamortized discount	(3,352,000)		(3,610,000)
Convertible Senior Notes | 2019 Notes
Unsecured Senior Notes
Interest expense	3,000,000.0	$ 3,200,000
Senior Notes
Unsecured Senior Notes
Unamortized discount	$ (11,038,000)		$ (12,144,000)