Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 3,769,495	$ 4,141,680
Weighted-average maturity (in months)	1 month 12 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 1,244,673
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	2,116,162
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 408,660