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Income Taxes - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2018 | Sep. 30, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |  |
Effective income tax rate |  |  | 18.30% | 26.20% |
Net income tax benefit |  |  | $ 15,454,000 |  |
Contribution made to qualified pension plans | $ 25,000,000 |  |  |  |
Federal statutory corporate income tax rate |  |  | 21.00% |  | 35.00%
Qualified Pension Plan [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Additional contribution made to qualified pension plans |  | $ 125,000,000 |  |  |
Contribution made to qualified pension plans |  | $ 150,000,000 |  |  |

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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