Derivative Financial Instruments and Hedging Activities - Summary of Outstanding Derivative Financial Instruments (Detail) - Natural Gas Swap Agreements	12 Months Ended
	Dec. 31, 2017 MMBTU $ / MMBTU	Dec. 31, 2016 MMBTU $ / MMBTU
Derivative [Line Items]
Commodity Derivatives Weighted Average Contract Price | $ / MMBTU	3.38	3.37
Commodity Derivatives Contract Volume | MMBTU	2,565,000	23,400,000
Commodity Derivatives Contract Period	2018	2017