Employee Retirement Plan - Schedule of Components of Net Periodic Benefit Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Components of net periodic benefit income:
Interest cost	$ (6.5)	$ (7.7)
Service cost	(0.9)	(1.1)
Expected return on assets	10.5	11.4
Amortization of actuarial gain (loss)	(0.7)	(0.7)
Total net periodic benefit income	2.4	1.9
OPEB Benefits [Member]
Components of net periodic benefit income:
Interest cost	(0.5)	(1.1)
Service cost	(0.2)	(0.2)
Amortization of prior service credit	3.1	2.3
Total net periodic benefit income	$ 2.4	$ 1.0