Note Borrowings (Federal funds purchased and repurchase Agreements) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	$ 374,405	$ 479,425
Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	374,405	479,425
US Treasury Securities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	265,117	52,519
US Treasury Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	69,030	32,700
US Treasury Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	53,781	0
US Treasury Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	142,306	19,819
Obligations of U.S. Government sponsored entities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	66,235	263,399
Obligations of U.S. Government sponsored entities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	0	95,720
Obligations of U.S. Government sponsored entities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	30,835	142,299
Obligations of U.S. Government sponsored entities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	35,400	25,380
Mortgage Backed Securities | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	31,383	152,220
Mortgage Backed Securities | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	0	39,108
Mortgage Backed Securities | After 30 to 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	0	58,552
Mortgage Backed Securities | After 90 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	31,383	54,560
Collateralized Mortgage Obligations | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	11,670	11,287
Collateralized Mortgage Obligations | Within 30 days | Secured Debt
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase liability	$ 11,670	$ 11,287