Derivative Financial Instruments and Hedging Activities - Summary of Outstanding Derivative Financial Instruments (Detail) - Natural Gas Swap Agreements	12 Months Ended
Dec. 31, 2016 MMBTU $ / MMBTU	Dec. 31, 2015 MMBTU $ / MMBTU
Derivative [Line Items]
Commodity Derivatives Weighted Average Contract Price \| $ / MMBTU	3.37	3.20
Commodity Derivatives Contract Volume \| MMBTU	23,400,000	1,800,000
Commodity Derivatives Contract Period	2017	2016

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