Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	May 04, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (2,638,000)	$ (2,865,000)	$ (6,267,000)	$ (6,267,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		2,372,000	895,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,145,000)	(822,000)
Net current-period other comprehensive income (loss)		227,000	73,000
Ending balance		$ (2,638,000)	(6,194,000)	$ (2,865,000)
Common Stock Shares Issued At The Market				0.7
Shares Issued Price Per Share - At The Market				$ 82.93
Proceeds From Issuance Of Common Stock - At The Market				$ 61,000,000.0
Stock Issued During Period, Shares, Dividend Reinvestment Plan	0.1	0.1		0.3
Stock Issued During Period, Value, Dividend Reinvestment Plan	$ 115.35	$ 109.05		$ 86.12
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan	1,000,000.0	1,900,000		22,000,000.0
Fees on Equity Issuance - At The Market				1,500,000
Maximum [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Proceeds from Stock Plans				75,000,000.0
Proceeds from Stock Plans				75,000,000.0
UnrealizedGainsLossesFromDefinedBenefitPensionAndPostretirementPlanItems [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5,083,000)	(5,146,000)	(4,933,000)	(4,933,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		63,000	66,000
Net current-period other comprehensive income (loss)		63,000	66,000
Ending balance		(5,083,000)	(4,867,000)	(5,146,000)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	2,475,000	2,309,000	(1,334,000)	(1,334,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		2,371,000	895,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,205,000)	(888,000)
Net current-period other comprehensive income (loss)		166,000	7,000
Ending balance		2,475,000	$ (1,327,000)	2,309,000
Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (30,000)	(28,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		1,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(3,000)
Net current-period other comprehensive income (loss)		(2,000)
Ending balance		$ (30,000)		$ (28,000)