Stockholders' Equity Accumulated Other Comprehensive Income (loss) - Reclassifications of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortization of defined benefit pension and postretirement plan items:
Income tax benefit		$ (23,538)	$ (21,114)	$ (21,123)
Accumulated other comprehensive loss		(2,865)	(6,267)	(6,713)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		4,838	1,289
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(1,436)	(728)
Other Comprehensive Income (Loss), Net of Tax		3,402	561
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect			(115)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Accumulated other comprehensive loss		2,309	(1,334)	(785)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		5,400	2,161
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(1,757)	(2,595)
Other Comprehensive Income (Loss), Net of Tax		3,643	(434)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect			(115)
Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges
Amortization of defined benefit pension and postretirement plan items:
Accumulated other comprehensive loss		(28)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		16
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(44)
Other Comprehensive Income (Loss), Net of Tax		(28)
AOCI Changes For Defined Benefit Pension And Postretirement Plans [Member]
Amortization of defined benefit pension and postretirement plan items:
Accumulated other comprehensive loss		(5,146)	(4,933)	(5,928)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(578)	(872)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		365	1,867
Other Comprehensive Income (Loss), Net of Tax		(213)	995
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of defined benefit pension and postretirement plan items:
Net of tax		1,436	728	(2,198)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of defined benefit pension and postretirement plan items:
Prior service cost	[1]	77	77	77
Net gain	[1]	(592)	(2,600)	(579)
Total before income taxes		(515)	(2,523)	(502)
Income tax benefit	[2]	150	656	63
Net of tax		(365)	(1,867)	(439)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Total before income taxes		2,428	3,623	(2,460)
Income tax benefit	[2]	(671)	(1,028)	701
Net of tax		1,757	2,595	(1,759)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges
Amortization of defined benefit pension and postretirement plan items:
Total before income taxes		60
Income tax benefit		(16)
Net of tax		44
Propane Swap Agreement [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	[3]	2,428	1,520	(647)
Natural Gas Futures [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	[3],[4]	0	2,096	(2,010)
Natural Gas Swaps [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	[3],[4]	0	$ 7	$ 197
Interest Rate Swap [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		$ 60

[1] 1) These amounts are included in the computation of net periodic benefits. See Note 17, Employee Benefit Plans, for additional details.

[2] (4) The income tax benefit is included in income tax expense in the accompanying consolidated statements of income.

[3] 2) These amounts are included in the effects of gains and losses from derivative instruments. See Note 8, Derivative Instruments, for additional details.

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