Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 1,738,839
Weighted-average maturity (in months)	2 months 9 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 444,530
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	1,239,074
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	5,235
Over one to two years
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 50,000