Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 3,539,459
Weighted-average maturity (in months)	2 months 15 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 276,113
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	3,041,052
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	122,294
Over 180 days to one year
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 100,000