Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 2,091	$ 3,165	$ 3,472
Interest Cost	7,480	5,486	5,003
Expected Return on Plan Assets	(10,689)	(10,883)	(9,693)
Prior Service Cost Amortization	356	356	301
Actuarial Loss Amortization	$ 0	5,507	8,089
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Nonoperating Income (Expense)
Sub-total	$ (762)	3,631	7,172
Amounts Capitalized or Deferred	1,598	(1,085)	(3,384)
NPBC Recognized	836	2,546	3,788
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1,493	2,890	3,034
Interest Cost	2,899	3,194	2,740
Expected Return on Plan Assets	(3,408)	(3,415)	(2,508)
Prior Service Cost Amortization	794	1,092	1,208
Actuarial Loss Amortization	$ (1,464)	1,020	1,045
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Nonoperating Income (Expense)
Sub-total	$ 314	4,781	5,519
Amounts Capitalized or Deferred	555	(2,388)	(3,136)
NPBC Recognized	869	2,393	2,383
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	250	273	354
Interest Cost	754	472	458
Prior Service Cost Amortization	55	55	56
Actuarial Loss Amortization	$ 0	794	1,489
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Nonoperating Income (Expense)
Sub-total	$ 1,059	1,594	2,357
Amounts Capitalized or Deferred	(327)	(472)	(712)
NPBC Recognized	$ 732	$ 1,122	$ 1,645