Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Amortization of pension and postretirement items:
Tax benefit	$ (14,084,000)	$ (11,793,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of pension and postretirement items:
Net of tax	(108,000)	(31,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of pension and postretirement items:
Prior service cost	17,000	14,000	[1]
Net loss	154,000	66,000	[1]
Total before tax	(137,000)	(52,000)
Tax benefit	55,000	21,000
Net of tax	(82,000)	(31,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of pension and postretirement items:
Total before tax	(43,000)	0
Tax benefit	17,000	0
Net of tax	(26,000)	0
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements	12,000	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	$ (55,000)	$ 0

[1] These amounts are included in the computation of net periodic costs (benefits). See Note 9, Employee Benefit Plans, for additional details.