Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Agreements [Line Items]
Unpaid Principal Balance	$ 4,433,769	$ 4,339,479
Fair value / Carrying Value	4,428,791	4,336,976
Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	2,287,990	3,361,179
Amortized Cost Balance	2,287,983	3,352,529
Fair value / Carrying Value	2,294,103	3,366,772
Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	2,145,779	978,300
Fair value / Carrying Value	2,134,688	970,204
Financing agreements with non-mark-to-market collateral provisions | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	793,754	1,156,899
Amortized Cost Balance	793,754	1,156,899
Fair value / Carrying Value	795,341	1,159,213
Agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Fair value / Carrying Value	858,100
Agreements with mark-to-market collateral provisions | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	858,066	1,338,077
Amortized Cost Balance	858,066	1,338,077
Fair value / Carrying Value	858,066	1,338,077
Agreements with mark-to-market collateral provisions | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	503,303
Fair value / Carrying Value	503,191
Securitized debt | Fair Value
Financing Agreements [Line Items]
Unpaid Principal Balance	636,170	866,203
Amortized Cost Balance	636,163	857,553
Fair value / Carrying Value	640,696	869,482
Securitized debt | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	1,412,476	648,300
Fair value / Carrying Value	1,405,685	645,027
Convertible senior notes | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance	230,000	230,000
Fair value / Carrying Value	$ 225,812	225,177
Senior notes | Carrying Value
Financing Agreements [Line Items]
Unpaid Principal Balance		100,000
Fair value / Carrying Value		$ 100,000