Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) claim	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Number of CBD cases outstanding	1
Accrual for Environmental Loss Contingencies, Significant Assumptions	The reserves are established based upon analyses conducted by the Company’s engineers and outside consultants and are adjusted from time to time based upon ongoing studies, the difference between actual and estimated costs, and other factors. The reserves may also be affected by rulings and negotiations with regulatory agencies.
Number of CBD cases on appeal	1
Undiscounted reserve balance | $	$ 6.0	$ 5.7