Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Unrealized gains, before tax	$ (8,954,000)		$ (1,182,000)	$ 108,508,000	$ (1,182,000)
Unrealized gain, after tax	(6,635,000)		$ (876,000)	80,404,000	$ (876,000)
Income recognized	1,000,000.0			1,400,000
Cash flow hedge gain (loss) to be reclassified				40,000,000
Cash flow hedge termination gain				5,000,000
Collateral requirements	159,800,000			159,800,000		$ 84,600,000
Variation margin, amount reducing derivative asset	187,000,000.0			187,000,000.0		113,700,000
Foreign Currency Exchange Forwards
Derivative [Line Items]
Notional amount	$ 25,100,000			$ 25,100,000		$ 32,900,000
Cash Flow Hedges
Derivative [Line Items]
Unrealized gains, before tax		$ 9,800,000
Unrealized gain, after tax		$ 7,300,000