Borrowings - Repurchase agreements accounted for as secured borrowings (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 233,091	$ 281,529
Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	233,091	281,529
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	185,669	237,621
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	56,485	138,689
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	47,531	79,374
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	81,653	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	46,527	27,324
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	26,527	6,859
Mortgage Backed Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	20,000	20,465
CMO | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	895	10,529
CMO | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 895	$ 10,529