Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) shares in Millions		1 Months Ended	3 Months Ended			9 Months Ended
		Oct. 31, 2021	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Amortization of pension and postretirement items:
Tax benefit			$ (2,993,000)		$ (3,502,000)	$ (20,563,000)	$ (16,082,000)
Derivative, Gain (Loss) on Derivative, Net			3,215,000		1,209,000	$ 11,272,000	4,997,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan		0.1		0.3		0.1
Stock Issued During Period, Value, Dividend Reinvestment Plan		$ 127.40		$ 86.12		$ 116.63
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan		$ 1,800,000		$ 22,000,000		6,600,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of pension and postretirement items:
Net of tax			447,000		43,000	2,872,000	979,000
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of pension and postretirement items:
Prior service cost	[1]		19,000		19,000	57,000	57,000
Net loss	[1]		(106,000)		(108,000)	(317,000)	(323,000)
Total before tax			(87,000)		(89,000)	(260,000)	(266,000)
Tax benefit			23,000		23,000	68,000	68,000
Net of tax			(64,000)		(66,000)	(192,000)	(198,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of pension and postretirement items:
Tax benefit			(188,000)		(32,000)	(1,156,000)	(437,000)
Net of tax			493,000		84,000	3,027,000	1,144,000
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					34,000		45,000
Tax benefit			(6,000)		(9,000)	(13,000)	(12,000)
Net of tax			18,000		25,000	37,000	33,000
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]		681,000		116,000	4,183,000	1,581,000
Interest Rate Swap [Member] | Derivatives designated as hedging instrument [Member] | Interest Expense [Member]
Amortization of pension and postretirement items:
Derivative, Gain (Loss) on Derivative, Net			$ 24,000		$ 34,000	$ 50,000	$ 45,000

[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.