Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Amortization of pension and postretirement items:
Tax benefit		$ (3,940)	$ (5,081)		$ (16,456)		$ (18,410)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of pension and postretirement items:
Net of tax		346	42		1,194		(147)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of pension and postretirement items:
Prior service cost	[1]	20	20		39		40
Net loss	[1]	(170)	166		339		333
Total before tax		(150)	(146)		(300)		(293)
Tax benefit		61	58		120		117
Net of tax		(89)	(88)		(180)		(176)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of pension and postretirement items:
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements		77	0	[2]	465	[2]	(322)	[2]
Total before tax		708	211		2,246		37
Tax benefit		(273)	(81)		(872)		(8)
Net of tax		435	130		1,374		29
Natural Gas Futures [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of pension and postretirement items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		$ 631	$ 211	[2]	$ 1,781	[2]	$ 359	[2]

[1] These amounts are included in the computation of net periodic costs (benefits). See Note 8, Employee Benefit Plans, for additional details.

[2] (2) These amounts are included in the effects of gains and losses from derivative instruments. See Note 11, Derivative Instruments, for additional details.