Derivative Financial Instruments (Offsetting Derivatives) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Derivative [Line Items]
Derivative instrument asset, Amount Offset, Total		$ (4,779)	$ (7,522)
Derivative instrument asset, Amount Offset		8,100	33,043
Derivative asset		133,158	292,291
Derivative Liability, Fair Value, Amount Offset Against Collateral		4,779	7,522
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		(1,871)	(4,485)
Derivative liability		7,163	347
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ (785)	(299)
Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	(785)	(432)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(1,247)	(329)
Derivative Liability, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	(1,247)	(389)
Hedge Accounting Positions [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	40,565	13,012
Derivative instrument asset, Amount Offset, Total	(40,565)	(12,879)
Derivative asset	0	133
Derivative Liability, Fair Value, Gross Liability	12,007	6,710
Derivative Liability, Fair Value, Amount Offset Against Collateral	12,007	6,650
Derivative liability	$ 0	60
Interest Rate Derivatives [Member] | Hedge Accounting Positions [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		11,855	39,641
Derivative Liability, Fair Value, Gross Liability		$ 3,153	$ 103