Derivative Activities - Offsetting of Derivative Assets - Offsetting of Derivative and Financial Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	$ 32,349	$ 8,303
Netting	(23,568)	(4,247)
Net amounts of liabilities presented in the balance sheet	8,781	4,056
Total
Gross amounts of recognized liabilities	36,261	10,043
Net amounts of liabilities presented in the consolidated balance sheet	12,693	5,796
Mortgage loans sold under agreements to repurchase
Net amounts of liabilities presented in the consolidated balance sheet	1,738,839
Debt Issuance Costs
Debt issuance costs, gross	799	672
Debt issuance costs	799	672
Gross amounts of recognized liabilities	1,739,638	2,381,538
Net amount of liabilities in the consolidated balance sheet	1,739,638	2,381,538
Total
Gross amounts of recognized liabilities	1,775,899	2,391,581
Gross amounts offset in the consolidated balance sheet	(23,568)	(4,247)
Net amounts of liabilities presented in the consolidated balance sheet	1,751,532	2,386,662
Net amount of liabilities in the consolidated balance sheet	12,693	5,796
Receivable from Counterparties
Total
Net amounts of liabilities presented in the consolidated balance sheet	1,752,331	2,387,334
Assets sold under agreements to repurchase
Mortgage loans sold under agreements to repurchase
Gross amounts of recognized liabilities	1,738,839	2,380,866
Net amounts of liabilities presented in the consolidated balance sheet	1,738,839	2,380,866
Net amounts of liabilities presented in the consolidated balance sheet	1,738,839	2,380,866
Debt Issuance Costs
Debt issuance costs	799	672
Net amount of liabilities in the consolidated balance sheet	1,739,638	2,381,538
Forward contracts | Purchases
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	29,569	1,272
Net amounts of liabilities presented in the balance sheet	29,569	1,272
Total
Gross amounts of recognized liabilities	29,569	1,272
Forward contracts | Sales
Derivatives: Subject to master netting arrangements:
Gross amounts of recognized liabilities	2,780	7,031
Net amounts of liabilities presented in the balance sheet	2,780	7,031
Total
Gross amounts of recognized liabilities	2,780	7,031
Interest rate lock commitments
Derivatives not subject to master netting arrangements
Gross amounts of recognized liabilities	3,912	1,740
Total
Net amounts of liabilities presented in the consolidated balance sheet	3,912	1,740
Net amount of liabilities in the consolidated balance sheet	$ 3,912	$ 1,740