Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plans
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	$ 916	$ 753	$ 2,748	$ 2,255
Interest Cost	1,343	1,273	4,029	3,819
Expected Return on Plan Assets	(1,694)	(1,562)	(5,082)	(4,684)
Prior Service Cost Amortization	54	52	162	158
Actuarial Loss Amortization	1,180	711	3,540	2,135
Sub-total	1,799	1,227	5,397	3,683
Amounts Capitalized and Deferred	(927)	(556)	(2,510)	(1,400)
Net Periodic Benefit Cost Recognized	872	671	2,887	2,283
Other Postretirement Benefit Plans, Defined Benefit
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	656	497	1,968	1,491
Interest Cost	729	671	2,187	2,015
Expected Return on Plan Assets	(273)	(230)	(819)	(690)
Prior Service Cost Amortization	420	421	1,260	1,261
Actuarial Loss Amortization	288	14	864	42
Sub-total	1,820	1,373	5,460	4,119
Amounts Capitalized and Deferred	(921)	(632)	(2,563)	(1,697)
Net Periodic Benefit Cost Recognized	899	741	2,897	2,422
Supplemental Employee Retirement Plans, Defined Benefit
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	28	15	84	43
Interest Cost	78	68	234	204
Prior Service Cost Amortization	2	2	8	8
Actuarial Loss Amortization	84	25	252	75
Sub-total	192	110	578	330
Net Periodic Benefit Cost Recognized	$ 192	$ 110	$ 578	$ 330