Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 848	$ 824	$ 2,544	$ 2,471
Interest Cost	1,469	1,514	4,407	4,543
Expected Return on Plan Assets	(1,946)	(1,826)	(5,838)	(5,479)
Prior Service Cost Amortization	81	66	243	197
Actuarial Loss Amortization	1,447	1,165	4,341	3,496
Sub-total	1,899	1,743	5,697	5,228
Amounts Capitalized and Deferred	(962)	(932)	(2,590)	(2,402)
Net Periodic Benefit Cost Recognized	937	811	3,107	2,826
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	733	744	2,199	2,231
Interest Cost	852	978	2,554	2,935
Expected Return on Plan Assets	(409)	(337)	(1,227)	(1,010)
Prior Service Cost Amortization	327	350	981	1,049
Actuarial Loss Amortization	346	524	1,038	1,573
Sub-total	1,849	2,259	5,545	6,778
Amounts Capitalized and Deferred	(930)	(1,226)	(2,557)	(3,418)
Net Periodic Benefit Cost Recognized	919	1,033	2,988	3,360
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	122	115	366	345
Interest Cost	101	98	303	294
Prior Service Cost Amortization	47	47	141	141
Actuarial Loss Amortization	121	74	365	222
Sub-total	391	334	1,175	1,002
Amounts Capitalized and Deferred	(113)	(99)	(339)	(297)
Net Periodic Benefit Cost Recognized	$ 278	$ 235	$ 836	$ 705