Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Amortization of pension and postretirement items:
Tax benefit		$ (4,220)	$ (3,425)	$ (18,304)	$ (15,218)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of pension and postretirement items:
Net of tax		(89)	(30)	(197)	(60)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of pension and postretirement items:
Prior service cost	[1]	17	15	34	30
Net loss	[1]	155	67	310	132
Total before tax		(138)	(52)	(276)	(102)
Tax benefit		55	21	111	41
Net of tax		(83)	(31)	(165)	(61)
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		(10)	2	(53)	2
Tax benefit		4	(1)	21	(1)
Net of tax		(6)	1	(32)	1
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements		(10)	2	2	2
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		$ 0	$ 0	$ (55)	$ 0

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