Borrowings - Repurchase agreements accounted for as secured borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 213,097	$ 281,529
Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	213,097	281,529
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	181,604	237,621
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	9,400	138,689
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	29,386	79,374
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	142,818	19,558
Obligations of U.S. Government sponsored entities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	6,055
Obligations of U.S. Government sponsored entities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	6,055
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	30,184	27,324
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	30,184	6,859
Mortgage Backed Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	20,465
CMO | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	1,309	10,529
CMO | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 1,309	$ 10,529