Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 848	$ 824
Interest Cost	1,469	1,514
Expected Return on Plan Assets	(1,946)	(1,819)
Prior Service Cost Amortization	81	66
Actuarial Loss Amortization	1,447	1,176
Sub-total	1,899	1,761
Amounts Capitalized and Deferred	(720)	(662)
Net Periodic Benefit Cost Recognized	1,179	1,099
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	733	744
Interest Cost	851	978
Expected Return on Plan Assets	(409)	(337)
Prior Service Cost Amortization	327	350
Actuarial Loss Amortization	346	524
Sub-total	1,848	2,259
Amounts Capitalized and Deferred	(742)	(1,037)
Net Periodic Benefit Cost Recognized	1,106	1,222
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	122	115
Interest Cost	101	98
Prior Service Cost Amortization	47	47
Actuarial Loss Amortization	122	74
Sub-total	392	334
Amounts Capitalized and Deferred	(113)	(99)
Net Periodic Benefit Cost Recognized	$ 279	$ 235