Short-Term Borrowings - Mortgage Loans Sold Under Agreement to Repurchase by Counterparty (Details) - Assets sold under agreements to repurchase $ in Thousands	Mar. 31, 2021 USD ($)
Credit Suisse First Boston Mortgage Capital LLC Tranche Two
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 2,069,444
Credit Suisse First Boston Mortgage Capital LLC Tranche One
Mortgage loans sold under agreement to repurchase
Amount at risk	592,940
Bank of America, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	501,873
JP Morgan Chase Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	282,086
JP Morgan
Mortgage loans sold under agreement to repurchase
Amount at risk	15,597
BNP Paribas
Mortgage loans sold under agreement to repurchase
Amount at risk	252,959
Barclays
Mortgage loans sold under agreement to repurchase
Amount at risk	94,629
Goldman Sachs
Mortgage loans sold under agreement to repurchase
Amount at risk	53,150
Citibank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	41,738
Morgan Stanley Bank
Mortgage loans sold under agreement to repurchase
Amount at risk	33,515
Royal Bank of Canada
Mortgage loans sold under agreement to repurchase
Amount at risk	30,200
Wells Fargo Bank, N.A.
Mortgage loans sold under agreement to repurchase
Amount at risk	$ 20,988