Retirement Plans - Amounts in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	$ (312.3)	$ (334.1)
Net prior service credit	(2.9)	2.8
Total	(315.2)	(331.3)
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	31.1	32.1
Net prior service credit	47.2	52.0
Total	$ 78.3	$ 84.1

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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-Publisher FASB
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Amount, before tax, of accumulated other comprehensive (income) loss for 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).

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