Components of Retirement Plan Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Pension Plans
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	$ 751	$ 893
Interest Cost	1,273	1,142
Expected Return on Plan Assets	(1,561)	(1,489)
Prior Service Cost Amortization	53	52
Actuarial Loss Amortization	712	1,059
Sub-total	1,228	1,657
Amounts Capitalized and Deferred	(344)	(616)
Net Periodic Benefit Cost Recognized	884	1,041
Other Postretirement Benefit Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	497	631
Interest Cost	672	612
Expected Return on Plan Assets	(230)	(180)
Prior Service Cost Amortization	420	425
Actuarial Loss Amortization	14	196
Sub-total	1,373	1,684
Amounts Capitalized and Deferred	(473)	(769)
Net Periodic Benefit Cost Recognized	900	915
Supplemental Employee Retirement Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	14	18
Interest Cost	68	60
Prior Service Cost Amortization	3	3
Actuarial Loss Amortization	25	46
Sub-total	110	127
Net Periodic Benefit Cost Recognized	$ 110	$ 127