Borrowings - Repurchase agreements accounted for as secured borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 86,834	$ 121,303
Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	86,834	121,303
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	74,193	116,454
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	14,665	67,157
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	14,732	39,318
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	44,796	9,979
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	11,864	4,046
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	2,129	3,778
Mortgage Backed Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	0	268
Mortgage Backed Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	9,735	0
CMO | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	777	803
CMO | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 777	$ 803