Accumulated Other Comprehensive Income (Loss) and Other Comprehensive Income (Loss) - Schedule of Components of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Other comprehensive income (loss), before income taxes	$ 28.7	$ (35.0)
Total tax (benefit) for the period	3.4	(3.6)
Other comprehensive income (loss), net of tax	25.3	(31.4)
Other comprehensive income (loss), before income taxes		5.1
Other comprehensive income (loss), before income taxes	(0.1)	3.9
Other comprehensive loss, before income taxes	(0.1)	9.0
Total tax expense for the period		3.4
Other comprehensive loss, net of tax	(0.1)	5.6
Amortization of actuarial loss and prior service credit	(2.4)	(1.6)
Other pension and OPEB plan adjustments	0.2	0.4
Other comprehensive income (loss), before income taxes	(2.2)	(1.2)
Total tax benefit for the period	(0.8)	(0.5)
Other comprehensive income (loss), net of tax	(1.4)	(0.7)
Other comprehensive income (loss), before income taxes	26.4	(27.2)
Total tax expense (benefit) for the period	2.6	(0.7)
Other comprehensive income (loss), net of tax	$ 23.8	$ (26.5)