Borrowings - Repurchase agreements accounted for as secured borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 86,470	$ 121,303
Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	86,470	121,303
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	74,606	116,454
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	14,574	67,157
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	20,394	39,318
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	39,638	9,979
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	11,295	4,046
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	1,427	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	9,868	0
CMO | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	569	803
CMO | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase
Repurchase liability	$ 569	$ 803