Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 4,141,680
Weighted-average maturity (in months)	2 months
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 715,059
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	3,157,444
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 269,177