Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
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	$ 15.3	$ 14.9	$ 18.3
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	$ 458.6	$ 483.3
Interest cost	$ 14.3	$ 14.0
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	$ (78.0)	$ (5.2)
Benefits paid	(33.2)	(33.5)
Benefit obligation, end of year	361.7	458.6	$ 483.3
Balance at beginning of period	478.0	510.7
Actual return on plan assets	(73.3)	0.8
Benefits paid	(33.2)	(33.5)
Balance at end of year	371.5	478.0	$ 510.7
Funded status of the plans	$ 9.8	$ 19.4
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	$ 31.0	$ 33.0
Interest cost	$ 1.0	$ 0.9
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	$ (4.5)	$ (0.1)
Benefits paid	(3.0)	(2.8)
Benefit obligation, end of year	24.5	31.0	$ 33.0
Contributions	3.0	2.8
Benefits paid	(3.0)	(2.8)
Funded status of the plans	$ (24.5)	$ (31.0)