Financing Agreements (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Jun. 30, 2020	Dec. 31, 2020
Financing Agreements [Line Items]
Agreement amount	$ 3,998,346,000		$ 4,339,479,000
Financial instruments	3,995,982,000		4,336,976,000
Financing Agreement
Financing Agreements [Line Items]
Cash pledged as collateral	5,200,000		7,200,000
Senior Secured Credit Agreement
Financing Agreements [Line Items]
Agreement amount		$ 500,000,000
Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Agreement amount	1,180,287,000		1,338,077,000
Financial instruments	1,180,287,000	$ 1,650,000,000	$ 1,338,077,000
Non Mark to Market Financing Facilities [Member]
Financing Agreements [Line Items]
Financial instruments	$ 837,800,000
Debt instrument term	2 years	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	$ 203,500,000
Minimum | Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Debt instrument term	1 month
Maximum | Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Debt instrument term	3 months