Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Sep. 30, 2020 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 1,092,694
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	460,264
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	410,222
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	192,579
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	48,231
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	62,630
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	27,646
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 33,343