Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 3,472	$ 3,322	$ 3,104
Interest Cost	5,003	5,776	6,484
Expected Return on Plan Assets	(9,693)	(9,019)	(8,475)
Prior Service Cost Amortization	301	320	320
Actuarial Loss Amortization	8,089	6,472	4,324
Sub-total	7,172	6,871	5,757
Amounts Capitalized or Deferred	(3,384)	(3,083)	(2,227)
NPBC Recognized	3,788	3,788	3,530
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	3,034	2,698	2,304
Interest Cost	2,740	3,121	3,426
Expected Return on Plan Assets	(2,508)	(2,063)	(1,645)
Prior Service Cost Amortization	1,208	1,210	1,213
Actuarial Loss Amortization	1,045	744	227
Sub-total	5,519	5,710	5,525
Amounts Capitalized or Deferred	(3,136)	(2,865)	(2,317)
NPBC Recognized	2,383	2,845	3,208
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	354	283	247
Interest Cost	458	549	567
Prior Service Cost Amortization	56	57	56
Actuarial Loss Amortization	1,489	1,036	628
Sub-total	2,357	1,925	1,498
Amounts Capitalized or Deferred	(712)	(579)	(430)
NPBC Recognized	$ 1,645	$ 1,346	$ 1,068