Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Derivative [Line Items]
Unrealized gains, before tax	$ (1,923)		$ 8,823		$ (30,980)	$ 117,462
Unrealized gain, after tax	(1,437)		$ 6,538		(22,620)	$ 87,039
Income recognized	3,700				5,300
Cash flow hedge gain (loss) to be reclassified					41,000
Cash flow hedge termination gain					10,000
Collateral requirements	118,200				118,200		$ 155,400
Variation margin, amount reducing derivative asset	114,000				114,000		162,700
Foreign Currency Exchange Forwards
Derivative [Line Items]
Notional amount	$ 19,900				$ 19,900		$ 24,100
Cash Flow Hedges
Derivative [Line Items]
Unrealized gains, before tax		$ 757		$ 9,800
Unrealized gain, after tax		$ 565		$ 7,300