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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) |

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