Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Agreements [Line Items]
Unpaid Principal Balance	$ 5,557,639	$ 4,339,479
Fair value / Carrying Value	5,550,808	4,336,976
Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	2,491,537	3,361,179
Amortized Cost Balance	2,491,531	3,352,529
Fair value / Carrying Value	2,496,584	3,366,772
Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	3,066,102	978,300
Fair value / Carrying Value	3,054,224	970,204
Financing agreements with non-mark-to-market collateral provisions | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	689,205	1,156,899
Amortized Cost Balance	689,205	1,156,899
Fair value / Carrying Value	690,583	1,159,213
Financing agreements with non-mark-to-market collateral provisions | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	157,784
Fair value / Carrying Value	157,366
Agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Fair value / Carrying Value	1,300,000
Agreements with mark-to-market collateral provisions | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	1,275,172	1,338,077
Amortized Cost Balance	1,275,172	1,338,077
Fair value / Carrying Value	1,275,172	1,338,077
Agreements with mark-to-market collateral provisions | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	1,155,964
Fair value / Carrying Value	1,155,820
Securitized debt | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	527,160	866,203
Amortized Cost Balance	527,154	857,553
Fair value / Carrying Value	530,829	869,482
Securitized debt | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	1,522,354	648,300
Fair value / Carrying Value	1,514,900	645,027
Convertible senior notes | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	230,000	230,000
Fair value / Carrying Value	$ 226,138	225,177
Senior notes | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance		100,000
Fair value / Carrying Value		$ 100,000