Fair-Value Measurements (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) from valuation of embedded derivatives			$ 19,582	$ (4,231)
Net Value	$ 2,373,928		2,373,928		$ 2,469,937
Fair value of senior unsecured notes	2,100,000		2,100,000		$ 2,500,000
Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) from valuation of embedded derivatives	$ 6,700	$ 8,000	$ 17,400	$ 300
Equity Volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input	0.50		0.50