Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020
Financing Agreements [Line Items]
Unpaid Principal Balance	$ 3,998,346	$ 4,339,479
Fair value / Carrying Value	3,995,982	4,336,976
Financing agreements with non-mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,039,205	1,156,899
Amortized Cost Balance	1,039,205	1,156,899
Fair value / Carrying Value	1,041,283	1,159,213
Financing agreements with mark-to-market collateral provisions
Financing Agreements [Line Items]
Unpaid Principal Balance	1,180,287	1,338,077
Amortized Cost Balance	1,180,287	1,338,077
Fair value / Carrying Value	1,180,287	1,338,077	$ 1,650,000
Securitized debt
Financing Agreements [Line Items]
Unpaid Principal Balance	748,717	866,203
Amortized Cost Balance	748,708	857,553
Fair value / Carrying Value	753,008	869,482
Total Financing agreements, at fair value
Financing Agreements [Line Items]
Unpaid Principal Balance	2,968,209	3,361,179
Amortized Cost Balance	2,968,200	3,352,529
Fair value / Carrying Value	2,974,578	3,366,772
Securitized debt
Financing Agreements [Line Items]
Unpaid Principal Balance	800,137	648,300
Fair value / Carrying Value	795,912	645,027
Convertible senior notes
Financing Agreements [Line Items]
Unpaid Principal Balance	230,000	230,000
Fair value / Carrying Value	225,492	225,177
Senior notes
Financing Agreements [Line Items]
Unpaid Principal Balance		100,000
Fair value / Carrying Value		100,000
Total Financing agreements at carrying value
Financing Agreements [Line Items]
Unpaid Principal Balance	1,030,137	978,300
Fair value / Carrying Value	$ 1,021,404	$ 970,204