Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Unrealized gains, before tax	$ 8,823,000		$ 0	$ 117,462,000	$ 0
Unrealized gain, after tax	6,538,000		$ 0	87,039,000	$ 0
Income recognized	270,000			390,000
Cash flow hedge gain (loss) to be reclassified				39,000,000
Cash flow hedge termination gain				5,000,000
Collateral requirements	159,300,000			159,300,000		$ 84,600,000
Variation margin, amount reducing derivative asset	198,500,000			198,500,000		113,700,000
Foreign Currency Exchange Forwards
Derivative [Line Items]
Notional amount	$ 35,100,000			$ 35,100,000		$ 32,900,000
Cash Flow Hedges
Derivative [Line Items]
Unrealized gains, before tax		$ 9,800,000
Unrealized gain, after tax		$ 7,300,000