Short-Term Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 10,856,677
Weighted-average maturity (in months)	5 months 3 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 2,712,890
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	5,075,748
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	428,639
Over 180 days to one year
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 2,639,400