Derivative Financial Instruments (Counterparty Credit Risk Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Derivative [Line Items]
Derivative instrument asset, Amount Offset		$ 8,100	$ 33,043
Derivative, Collateral, Right to Reclaim Cash	$ 302,000
Credit Derivative, Maximum Exposure, Undiscounted	296,300
Valuation, Cost Approach [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash	34,300
Valuation, Market Approach [Member]
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	39,500
CME SWAPS MARKETS (CME) [Member]
Derivative [Line Items]
Derivative instrument asset, Amount Offset	79,000
Derivative, Collateral, Right to Reclaim Cash	$ 252,000