Derivative Activities - Offsetting of Derivative Assets - Offsetting of Derivative and Financial Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	$ 29,132	$ 26,977
Netting	(13,477)	(25,082)
Net amounts of liabilities presented in the balance sheet	15,655	1,895
Total
Gross amounts of recognized liabilities	31,315	28,146
Net amounts of liabilities presented in the consolidated balance sheet	17,838	3,064
Mortgage loans sold under agreements to repurchase
Net amounts of liabilities presented in the consolidated balance sheet	2,152,588
Debt Issuance Costs
Debt issuance costs, gross	(650)	(1,341)
Debt issuance costs	(650)	(1,341)
Gross amounts of recognized liabilities	2,151,938	1,933,859
Net amount of liabilities in the consolidated balance sheet	2,151,938	1,933,859
Total
Gross amounts of recognized liabilities	2,183,253	1,962,005
Gross amounts offset in the consolidated balance sheet	(13,477)	(25,082)
Net amounts of liabilities presented in the consolidated balance sheet	2,170,426	1,938,264
Net amount of liabilities in the consolidated balance sheet	17,838	3,064
Receivable from Counterparties
Total
Net amounts of liabilities presented in the consolidated balance sheet	2,169,776	1,936,923
Assets sold under agreements to repurchase
Mortgage loans sold under agreements to repurchase
Gross amounts of recognized liabilities	2,152,588	1,935,200
Net amounts of liabilities presented in the consolidated balance sheet	2,152,588	1,935,200
Net amounts of liabilities presented in the consolidated balance sheet	2,152,588	1,935,200
Debt Issuance Costs
Debt issuance costs	(650)	(1,341)
Net amount of liabilities in the consolidated balance sheet	2,151,938	1,933,859
Forward contracts | Purchases
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	3,170	215
Net amounts of liabilities presented in the balance sheet	3,170	215
Total
Gross amounts of recognized liabilities	3,170	215
Forward contracts | Sales
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	25,962	26,762
Net amounts of liabilities presented in the balance sheet	25,962	26,762
Total
Gross amounts of recognized liabilities	25,962	26,762
Interest rate lock commitments
Derivatives not subject to master netting arrangements
Gross amounts of recognized liabilities	2,183	1,169
Total
Net amounts of liabilities presented in the consolidated balance sheet	2,183	1,169
Net amount of liabilities in the consolidated balance sheet	$ 2,183	$ 1,169