Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 19.8	$ 21.9	$ 24.9
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	146.1	132.2
Plan amendment	0.3
Interest cost	3.7	3.9
Actuarial losses (gains)	(8.6)	3.2
Benefits paid	(3.9)	(5.8)
Foreign currency translation	(8.3)	12.6
Disposition of benefit obligations with the sale of Chaucer	(129.3)
Benefit obligation, end of year		146.1	132.2
Fair value of plan assets, beginning of period	144.6	116.5
Actual return on plan assets	(5.8)	14.4
Contributions		7.7
Benefits paid	(3.9)	(5.8)
Foreign currency translation	(8.1)	11.8
Disposition of plan assets with the sale of Chaucer	(126.8)
Fair value of plan assets, end of year		144.6	116.5
Funded status of the plans		(1.5)
Pension Plans [Member] | Qualified Plan [Member] | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	501.2	506.8
Interest cost	18.4	20.4
Actuarial losses (gains)	(17.5)	7.3
Benefits paid	(32.6)	(33.3)
Benefit obligation, end of year	469.5	501.2	506.8
Fair value of plan assets, beginning of period	460.1	453.3
Actual return on plan assets	(16.8)	40.1
Contributions	40.0
Benefits paid	(32.6)	(33.3)
Fair value of plan assets, end of year	450.7	460.1	453.3
Funded status of the plans	(18.8)	(41.1)
Pension Plans [Member] | Nonqualified Plan | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	36.6	38.0
Interest cost	1.4	1.5
Actuarial losses (gains)	(1.3)	0.1
Benefits paid	(2.9)	(3.0)
Benefit obligation, end of year	33.8	36.6	$ 38.0
Contributions	2.9	3.0
Benefits paid	(2.9)	(3.0)
Funded status of the plans	$ (33.8)	$ (36.6)