Reconciliation of Fair Value Measurements Using Significant Unobservable Inputs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Plan assets at beginning of year	$ 775.3
Plan assets at end of year	828.9	$ 775.3
Significant Non-observable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets at beginning of year	93.6	98.7
Relating to assets still held at the reporting date	1.9	2.3
Relating to assets sold during the period	(1.4)	(0.8)
Purchases, sales and settlements	(3.2)	(6.6)
Plan assets at end of year	90.9	93.6
Real estate funds | Significant Non-observable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets at beginning of year	80.4	77.8
Relating to assets still held at the reporting date	1.9	2.6
Plan assets at end of year	82.3	80.4
Private equity | Significant Non-observable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets at beginning of year	13.2	20.9
Relating to assets still held at the reporting date		(0.3)
Relating to assets sold during the period	(1.4)	(0.8)
Purchases, sales and settlements	(3.2)	(6.6)
Plan assets at end of year	$ 8.6	$ 13.2