Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [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
Interest cost	$ 21.9	$ 15.3	$ 14.9
Qualified Plan [Member]
Defined Benefit Plan Disclosure [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
Benefit obligation, beginning of period	$ 361.7	$ 458.6
Interest cost	$ 20.5	$ 14.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Cost and Expense, Operating	Other Cost and Expense, Operating
Actuarial gains	$ (1.6)	$ (78.0)
Benefits paid	(32.2)	(33.2)
Benefit obligation, end of year	348.4	361.7	$ 458.6
Balance at beginning of period	371.5	478.0
Actual return on plan assets	23.7	(73.3)
Benefits paid	(32.2)	(33.2)
Balance at end of year	363.0	371.5	$ 478.0
Funded status of the plans	$ 14.6	$ 9.8
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [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
Benefit obligation, beginning of period	$ 24.5	$ 31.0
Interest cost	$ 1.4	$ 1.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Cost and Expense, Operating	Other Cost and Expense, Operating
Actuarial gains	$ (0.5)	$ (4.5)
Benefits paid	(2.8)	(3.0)
Benefit obligation, end of year	22.6	24.5	$ 31.0
Contributions	2.8	3.0
Benefits paid	(2.8)	(3.0)
Funded status of the plans	$ (22.6)	$ (24.5)