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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Amortization of pension and postretirement items:
Tax benefit		$ (1,983)	$ (3,379)	$ (12,580)	$ (13,002)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of pension and postretirement items:
Net of tax		114	(14)	936	(81)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of pension and postretirement items:
Prior service cost	[1]	19	19	38	39
Net loss	[1]	(108)	(163)	(215)	(328)
Total before tax		(89)	(144)	(177)	(289)
Tax benefit		23	37	45	76
Net of tax		(66)	(107)	(132)	(213)
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		238	127	1,465	171
Tax benefit		(66)	(34)	(405)	(39)
Net of tax		172	93	1,060	132
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges [Member]
Amortization of pension and postretirement items:
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements				11
Total before tax		11	0	11	0
Tax benefit		(3)	0	(3)	0
Net of tax		8	0	8	0
Propane Swap Agreement [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 Adjustments AOCI Swap Agreements	[2]	238	252	1,465	858
Natural Gas Swaps [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	[2],[3]				11
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	[2],[3]	$ 0	$ (125)	$ 0	$ (698)

[1] These amounts are included in the computation of net periodic costs (benefits). See Note 10, Employee Benefit Plans, for additional details.

[2] These amounts are included in the effects of gains and losses from derivative instruments. See Note 13, Derivative Instruments, for additional details.

[3] PESCO's results are reflected as discontinued operations in our condensed consolidated statements of income.