Pension Plans (Summary for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	us-gaap:DomesticPlanMember	us-gaap:DomesticPlanMember	us-gaap:DomesticPlanMember
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 14.4	$ 16.0
Less: Assets transferred to Level 1 investments	(2.5)	(2.0)
Return on plan assets related to assets still held	0.4	0.4
Balance at end of year	$ 12.3	$ 14.4	$ 16.0