Financing Agreements (Financing Agreements, at Fair Values Narrative) (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 26, 2020	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Financing Agreements [Line Items]
Agreement amount		$ 4,339,479		$ 10,044,844
Financial instruments		4,336,976		10,031,606
Financing Agreement
Financing Agreements [Line Items]
Cash collateral pledged		7,200		$ 25,200
Senior secured credit agreement
Financing Agreements [Line Items]
Agreement amount			$ 500,000
Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Agreement amount		1,338,077
Financial instruments	$ 1,650,000	1,338,077
Non Mark to Market Financing Facilities
Financing Agreements [Line Items]
Financial instruments		886,100
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		$ 273,100
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