Retirement Plans and Other Post-Retirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	$ 1.7	$ (5.2)
Plan amendments		(0.2)
Recognized prior service costs		(0.2)
Recognized actuarial losses	(0.7)	(78.1)
Total recognized in other comprehensive (income) loss	1.0	(83.7)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	2.9	(5.0)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	0.9	(0.3)
Recognized actuarial losses	0.8	0.9
Total recognized in other comprehensive (income) loss	1.7	$ 0.6
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 0.6

Defined benefit plan amortization of gains losses including settlements.

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- Definition

Defined benefit plan amounts recognized in other comprehensive (income) loss.

+ References

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- Definition

Amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

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- Definition

Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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