Note Borrowings (Federal funds purchased and repurchase Agreements) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	$ 306,911	$ 390,921
Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	306,911	390,921
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	258,434	279,373
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	147,318	148,516
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	64,413	87,357
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	46,703	43,500
Obligations of U.S. Government sponsored entities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	16,152	66,056
Obligations of U.S. Government sponsored entities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	5,152	30,656
Obligations of U.S. Government sponsored entities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	5,000	19,463
Obligations of U.S. Government sponsored entities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	6,000	15,937
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	22,642	31,383
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	22,642	31,383
Collateralized Mortgage Obligations | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	9,683	14,109
Collateralized Mortgage Obligations | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	$ 9,683	$ 14,109