Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (2,865,000)			$ (2,865,000)	$ (6,267,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					5,848,000	2,741,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(2,425,000)	(936,000)
Net current-period other comprehensive income (loss)	$ 3,196,000			$ 1,732,000	3,423,000	1,805,000
Ending balance	558,000		$ (2,865,000)	(4,462,000)	$ 558,000	(4,462,000)
Stock Issued During Period, Shares, Dividend Reinvestment Plan			0.3		0.1
Stock Issued During Period, Value, Dividend Reinvestment Plan			$ 86.12		$ 113.51
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan			$ 22,000,000		$ 4,500,000
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
Stock Issued During Period, Shares, Dividend Reinvestment Plan			0.3		0.1
Stock Issued During Period, Value, Dividend Reinvestment Plan			$ 86.12		$ 113.51
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan			22,000,000		4,500,000
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
Proceeds from Stock Plans			75,000,000
UnrealizedGainsLossesFromDefinedBenefitPensionAndPostretirementPlanItems [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(5,146,000)			(5,146,000)	(4,933,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					128,000	132,000
Net current-period other comprehensive income (loss)					128,000	132,000
Ending balance	(5,018,000)		(5,146,000)	(4,801,000)	(5,018,000)	(4,801,000)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		2,309,000			2,309,000	(1,334,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					5,825,000	2,770,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(2,534,000)	(1,060,000)
Net current-period other comprehensive income (loss)					3,291,000	1,710,000
Ending balance	5,600,000		2,309,000	376,000	5,600,000	376,000
Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (28,000)			(28,000)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					23,000	(29,000)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(19,000)	(8,000)
Net current-period other comprehensive income (loss)					4,000	(37,000)
Ending balance	$ (24,000)		$ (28,000)	$ (37,000)	$ (24,000)	$ (37,000)