Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ / shares in Units, shares in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (3,629,000)			$ (6,267,000)	$ (6,713,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				3,617,000	1,700,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(979,000)	(275,000)
Net current-period other comprehensive income (loss)		$ 833,000	$ 344,000	2,638,000	1,425,000
Ending balance		$ (3,629,000)	(5,403,000)	$ (3,629,000)	(5,403,000)
Common Stock Shares Issued At The Market		100		100
Shares Issued Price Per Share - At The Market		$ 83.21		$ 83.21
Proceeds From Issuance Of Common Stock - At The Market		$ 10,200,000
Fees on Equity Issuance - At The Market		$ 400,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan		100
Stock Issued During Period, Value, Dividend Reinvestment Plan		$ 84.90
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan		$ 9,000,000.0
Common Stock Shares Issued At The Market		100		100
Shares Issued Price Per Share - At The Market		$ 83.21		$ 83.21
Proceeds From Issuance Of Common Stock - At The Market		$ 10,200,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan		100
Stock Issued During Period, Value, Dividend Reinvestment Plan		$ 84.90
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan		9,000,000.0
Fees on Equity Issuance - At The Market		400,000
Subsequent Event [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Common Stock Shares Issued At The Market	600
Shares Issued Price Per Share - At The Market	$ 82.88
Proceeds From Issuance Of Common Stock - At The Market	$ 50,800,000
Fees on Equity Issuance - At The Market	$ 1,100,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan	100
Stock Issued During Period, Value, Dividend Reinvestment Plan	$ 86.99
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan	$ 13,000,000.0
Common Stock Shares Issued At The Market	600
Shares Issued Price Per Share - At The Market	$ 82.88
Proceeds From Issuance Of Common Stock - At The Market	$ 50,800,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan	100
Stock Issued During Period, Value, Dividend Reinvestment Plan	$ 86.99
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan	13,000,000.0
Fees on Equity Issuance - At The Market	1,100,000
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	(4,735,000)			$ (4,933,000)	(5,928,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				0	(80,000)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				198,000	312,000
Net current-period other comprehensive income (loss)				198,000	232,000
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect					0
Ending balance		(4,735,000)	(5,696,000)	(4,735,000)	(5,696,000)
Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	1,116,000			(1,334,000)	(785,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				3,594,000	1,780,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(1,144,000)	(587,000)
Net current-period other comprehensive income (loss)				2,450,000	1,193,000
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect					(115,000)
Ending balance		1,116,000	$ 293,000	1,116,000	293,000
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect					$ (115,000)
Accumulated (Gain) Loss from Interest Rate Swap Cash Flows Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (10,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				23,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(33,000)
Net current-period other comprehensive income (loss)				(10,000)
Ending balance		$ (10,000)		$ (10,000)