Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 1,935,200
Weighted-average maturity (in months)	2 months 27 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 397,374
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	1,397,080
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	746
Over one to two years
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 140,000