Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 848	$ 823	$ 1,696	$ 1,647
Interest Cost	1,469	1,515	2,938	3,029
Expected Return on Plan Assets	(1,946)	(1,834)	(3,892)	(3,653)
Prior Service Cost Amortization	81	65	162	131
Actuarial Loss Amortization	1,447	1,155	2,894	2,331
Sub-total	1,899	1,724	3,798	3,485
Amounts Capitalized and Deferred	(908)	(808)	(1,628)	(1,470)
Net Periodic Benefit Cost Recognized	991	916	2,170	2,015
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	733	743	1,466	1,487
Interest Cost	851	979	1,702	1,957
Expected Return on Plan Assets	(409)	(336)	(818)	(673)
Prior Service Cost Amortization	327	349	654	699
Actuarial Loss Amortization	346	525	692	1,049
Sub-total	1,848	2,260	3,696	4,519
Amounts Capitalized and Deferred	(885)	(1,154)	(1,627)	(2,191)
Net Periodic Benefit Cost Recognized	963	1,106	2,069	2,328
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	122	115	244	230
Interest Cost	101	98	202	196
Prior Service Cost Amortization	47	47	94	94
Actuarial Loss Amortization	122	74	244	148
Sub-total	392	334	784	668
Amounts Capitalized and Deferred	(113)	(99)	(226)	(198)
Net Periodic Benefit Cost Recognized	$ 279	$ 235	$ 558	$ 470