Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Sep. 30, 2019 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 1,313,083
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	231,656
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	82,942
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	40,511
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	79,142
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	12,363
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	9,804
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 41,279