Employee Benefit Plans - Projected future postretirement benefit payments and future receipts from the federal subsidy (Detail) - Other Postretirement Benefits Plan [Member] $ in Millions	Dec. 31, 2020 USD ($)
Defined benefit plan, expected future benefit payment
2021	$ 2.0
2022	1.7
2023	1.5
2024	1.4
2025	1.3
Next five years	4.4
Defined benefit plan, expected future receipts from federal subsidy
Next five years	$ 0.1

Defined Benefit Plan, Expected Future Receipts From Federal Subsidy [Abstract]

No definition available.

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- Definition

Defined benefit plan expected future receipts from federal subsidy after year five.

+ References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

+ References

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in fifth fiscal year following current fiscal year.

+ References

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in fourth fiscal year following current fiscal year.

+ References

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in third fiscal year following current fiscal year.

+ References

+ Details

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- Definition

Amount of benefit for defined benefit plan expected to be paid in second fiscal year following current fiscal year.

+ References

+ Details

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- Details

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