Borrowings - Maturities of Outstanding Advances Under Repurchase Agreements (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 2,152,588
Weighted-average maturity (in months)	2 months 15 days
Within 30 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 518,972
Over 30 to 90 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	1,334,389
Over 90 to 180 days
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	44,227
Over one to two years
Mortgage loans sold under agreement to repurchase
Unpaid principal balance	$ 255,000