Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 7,267,046	$ 4,141,680
Weighted-average maturity (in months)	2 months
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 1,490,837
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	5,365,208
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	361,001
Over 180 days to one year
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 50,000