Postretirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Liability related to funded defined-benefit pension plans	$ 386.0	$ 403.0
Cash contributions made to benefit plans	13.6
Insurance cash value and Rabbi Trust investments supporting supplemental pension plan liability	78.0	74.0
Non-qualified supplemental retirement pension plan liability	$ 73.0	$ 71.0