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, 2020	Dec. 31, 2019
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
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 22.9	$ 25.8
Less: Assets transferred to Level 1 investments	(4.3)	(3.5)
Actual return on plan assets related to assets still held	0.5	0.6
Balance at end of year	$ 19.1	$ 22.9