Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [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
Interest cost	$ 18.3	$ 21.3	$ 19.8
Qualified Plan [Member]
Defined Benefit Plan Disclosure [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	country:US
Benefit obligation, beginning of period	$ 472.5	$ 469.5
Interest cost	17.1	19.9
Actuarial losses (gains)	25.9	20.4
Benefits paid	(32.2)	(37.3)
Benefit obligation, end of year	483.3	472.5	$ 469.5
Balance at beginning of period	487.6	450.7
Actual return on plan assets	55.3	64.2
Contributions		10.0
Benefits paid	(32.2)	(37.3)
Balance at end of year	510.7	487.6	$ 450.7
Funded status of the plans	$ 27.4	$ 15.1
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [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	country:US
Benefit obligation, beginning of period	$ 31.7	$ 33.8
Interest cost	1.2	1.4
Actuarial losses (gains)	3.0	(0.6)
Benefits paid	(2.9)	(2.9)
Benefit obligation, end of year	33.0	31.7	$ 33.8
Contributions	2.9	2.9
Benefits paid	(2.9)	(2.9)
Funded status of the plans	$ (33.0)	$ (31.7)