Pension and Post-Retirement Benefits - Components of Net Periodic Benefit Costs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net periodic benefit (income) costs:
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments			$ (1.5)
Defined Benefit Plan, Net Periodic Benefit Cost	(5.4)	(0.4)	3.9
Pension Plans
Net periodic benefit (income) costs:
Service cost	0.8	0.9	1.6
Interest cost	30.1	27.6	30.0
Expected return on plan assets	(41.3)	(38.3)	(35.2)
Amortization of actuarial loss	4.5	8.6	8.1
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	0	0	(1.5)
Defined Benefit Plan Net Periodic Benefit Cost Before Portion Allocated To Affiliate	(5.9)	(1.2)	3.0
Defined Benefit Plan Net Periodic Benefit Cost Portion Allocated To Affiliate	(0.1)	(0.1)	(0.1)
Defined Benefit Plan, Net Periodic Benefit Cost	(6.0)	(1.3)	2.9
Other Post-Retirement Benefit Plans
Net periodic benefit (income) costs:
Service cost	0	0	0
Interest cost	0.5	0.6	0.7
Expected return on plan assets	0	0	0
Amortization of actuarial loss	0.1	0.4	0.3
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost Before Portion Allocated To Affiliate	0.6	1.0	1.0
Defined Benefit Plan Net Periodic Benefit Cost Portion Allocated To Affiliate	0	(0.1)	0
Defined Benefit Plan, Net Periodic Benefit Cost	$ 0.6	$ 0.9	$ 1.0