Fair-Value Measurements (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized (loss) gain from valuation of embedded derivatives	$ (17,599)	$ 31,118
Carrying value of senior unsecured notes	2,671,262		$ 2,750,016
Fair value of senior unsecured notes	2,700,000		$ 2,700,000
Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized (loss) gain from valuation of embedded derivatives	$ (18,400)	$ 32,500
Equity Volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input	0.50