Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Mar. 31, 2019 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 1,165,339
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	26,541
Deutsche Bank AG
Mortgage loans sold under agreement to repurchase
Amount at risk	90,911
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	14,405
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	5,237
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	4,360
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	16,691
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	2,631
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 4,828