Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jun. 26, 2020	Dec. 31, 2019
Financing Agreements [Line Items]
Unpaid Principal Balance	$ 4,864,554			$ 10,044,844
Fair value / Carrying Value	4,851,121			10,031,606
Financing agreements with non-mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,723,959
Amortized Cost Balance	1,723,959
Fair value / Carrying Value	1,727,407
Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,489,097
Amortized Cost Balance	1,489,097
Fair value / Carrying Value	1,490,271		$ 1,650,000
Senior secured credit agreement
Financing Agreements [Line Items]
Unpaid Principal Balance	481,250	$ 500,000
Amortized Cost Balance	462,923
Fair value / Carrying Value	473,993
Securitized debt
Financing Agreements [Line Items]
Unpaid Principal Balance	389,051			573,900
Amortized Cost Balance	380,407
Fair value / Carrying Value	388,790			570,952
Total Financing agreements, at fair value
Financing Agreements [Line Items]
Unpaid Principal Balance	4,083,357
Amortized Cost Balance	4,056,386
Fair value / Carrying Value	4,080,461
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	451,197
Fair value / Carrying Value	448,893
Convertible senior notes
Financing Agreements [Line Items]
Unpaid Principal Balance	230,000			230,000
Fair value / Carrying Value	224,867			223,971
Senior notes
Financing Agreements [Line Items]
Unpaid Principal Balance	100,000			100,000
Fair value / Carrying Value	96,900			$ 96,862
Total Financing agreements at carrying value
Financing Agreements [Line Items]
Unpaid Principal Balance	781,197
Fair value / Carrying Value	$ 770,660