Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 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		$ (387)
Pretax total		3,251	$ (3,365)
Amortization of net actuarial loss		(1,690)
Amortization of prior service credit		326
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of debt securities		4,655	4,671
Change in unrealized gains (losses) on debt securities for which credit-related portion was recognized in earnings		(40)	(7,493)
Pretax total		4,615	(2,822)
Tax effect		(1,123)	854
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Pretax total	[1]	(1,364)	(543)
Tax effect	[1]	362	224
Amortization of net actuarial loss	[1]	(1,690)	(1,320)
Amortization of prior service credit	[1]	$ 326	$ 777

[1] Amounts are components of net periodic cost.  See Note 11 Employee Benefit Plans for additional details