Employee Postretirement Benefits - Schedule of Postretirement Benefit Amounts Recognized in Accumulated Other Comprehensive Income, Pretax and Regulatory Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 236.1	$ 208.2
Prior service cost (credit)	0.7	0.0
Amount recognized	236.8	208.2
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	50.5	47.2
Prior service cost (credit)	(15.1)	(17.0)
Amount recognized	$ 35.4	$ 30.2