Defined Contribution and Defined Benefit Retirement - Schedule of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Detail) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Net actuarial gain (loss)	$ (27.0)	$ 4.0	$ (38.0)
Amortization of unrecognized prior service cost (credit)	0.2	0.3	0.3
Amortization of unrecognized net actuarial loss	15.2	15.2	13.3
Total	$ (11.6)	$ 19.5	$ (24.4)