Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Change in unrealized gains (losses) on debt securities for which credit-related portion was recognized in earnings				$ (365)
Pretax total		$ 5,561	$ 6,083	13,725	$ 3,545
Amortization of net actuarial loss				(5,070)
Amortization of prior service credit				978
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains on sales of debt securities		6,937	6,694	17,888	12,749
Change in unrealized gains (losses) on debt securities for which credit-related portion was recognized in earnings		(12)	(67)	(71)	(7,574)
Pretax total		6,925	6,627	17,817	5,175
Tax effect		(1,726)	(1,322)	(4,357)	(1,278)
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Pretax total	[1]	(1,364)	(544)	(4,092)	(1,630)
Tax effect	[1]	363	145	1,086	433
Amortization of net actuarial loss	[1]	(1,690)	(1,321)	(5,070)	(3,961)
Amortization of prior service credit	[1]	$ 326	$ 777	$ 978	$ 2,331

[1] Amounts are components of net periodic cost.  See Note 13 Employee Benefit Plans for additional details.