Financing Agreements (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Financing Agreements [Line Items]
Agreement amount		$ 4,433,769	$ 4,339,479
Financial instruments		4,428,791	4,336,976
Financing Agreement
Financing Agreements [Line Items]
Cash pledged as collateral		8,800	$ 7,200
Agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Financial instruments		858,100
Non Mark to Market Financing Facilities
Financing Agreements [Line Items]
Financial instruments	$ 1,650,000	$ 639,700
Debt instrument term		2 years
Debt instrument, term, extension		3 years
Non Mark to Market Financing Facilities on Rehabilitation Loans
Financing Agreements [Line Items]
Long-term debt, term	2 years
Debt outstanding		$ 155,700
Minimum | Agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Debt instrument term	1 month
Maximum | Agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Debt instrument term	3 months