Derivative Financial Instruments (Counterparty Credit Risk Narrative) (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Maximum Length of Time Hedged in Cash Flow Hedge	4 years
Derivative instrument asset, Amount Offset	$ 32,159	$ 8,100
Derivative, Collateral, Right to Reclaim Cash	349,500
Credit Derivative, Maximum Exposure, Undiscounted	346,100
Derivative Credit Risk Valuation Adjustment, Derivative Assets (Liabilities)	4,200
Cash and Due from Banks [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash	33,200
Valuation, Market Approach [Member]
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	40,000
CME [Member]
Derivative [Line Items]
Derivative instrument asset, Amount Offset	82,800
Derivative, Collateral, Right to Reclaim Cash	$ 294,600