Pension Plans (Summary for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 46.6
Balance at end of year	39.6	[1]
Pension Plans [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	25.7
Plus: Assets transferred from investments measured at fair value using NAV	2.1
Less: Assets transferred to Level 1 investments	(2.7)
Actual return on plan assets related to assets still held	0.7
Balance at end of year	$ 25.8

[1] There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018.