Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Sep. 30, 2018 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 1,290,904
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	33,078
Deutsche Bank AG
Mortgage loans sold under agreement to repurchase
Amount at risk	84,185
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	17,796
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	4,656
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	4,937
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	4,384
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	1,704
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 783