Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jun. 26, 2020	Dec. 31, 2019
Financing Agreements [Line Items]
Unpaid Principal Balance	$ 4,339,479		$ 10,044,844
Fair value / Carrying Value	4,336,976		10,031,606
Financing agreements with non-mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,156,899
Amortized Cost Balance	1,156,899
Fair value / Carrying Value	1,159,213
Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,338,077
Amortized Cost Balance	1,338,077
Fair value / Carrying Value	1,338,077	$ 1,650,000
Securitized debt
Financing Agreements [Line Items]
Unpaid Principal Balance	866,203		573,900
Amortized Cost Balance	857,553
Fair value / Carrying Value	869,482		570,952
Total Financing agreements, at fair value
Financing Agreements [Line Items]
Unpaid Principal Balance	3,361,179
Amortized Cost Balance	3,352,529
Fair value / Carrying Value	3,366,772
Repurchase agreements
Financing Agreements [Line Items]
Unpaid Principal Balance			9,140,944
Fair value / Carrying Value			9,139,821
Securitized debt
Financing Agreements [Line Items]
Unpaid Principal Balance	648,300
Fair value / Carrying Value	645,027
Convertible senior notes
Financing Agreements [Line Items]
Unpaid Principal Balance	230,000		230,000
Fair value / Carrying Value	225,177		223,971
Senior Notes
Financing Agreements [Line Items]
Unpaid Principal Balance	100,000		100,000
Fair value / Carrying Value	100,000		$ 96,862
Total Financing agreements at carrying value
Financing Agreements [Line Items]
Unpaid Principal Balance	978,300
Fair value / Carrying Value	$ 970,204