Short-Term Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Jun. 30, 2021 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 2,126,158
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	724,309
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	397,300
JP Morgan Chase Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	207,772
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	1,891
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	124,512
Barclays
Mortgage loans sold under agreement to repurchase
Amount at risk	41,978
Goldman Sachs
Mortgage loans sold under agreement to repurchase
Amount at risk	77,468
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	11,835
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	44,701
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	55,641
Wells Fargo Bank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 20,750