Derivative Financial Instruments - Summary of Our Commodity Swap Contracts Outstanding (Detail) - Swap [Member]	3 Months Ended
	Mar. 31, 2017 MMBTU $ / MillionsofBTU-MMBTU
Product [Member]
Derivative [Line Items]
Derivative, Type of Instrument	Swap
Derivative, Swap Type, Fixed Price | $ / MillionsofBTU-MMBTU	2.162
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	150
Product [Member] | Contract Start Date [Member]
Derivative [Line Items]
Derivative, Inception Date	May 01, 2017
Product [Member] | Contract End Date [Member]
Derivative [Line Items]
Derivative, Maturity Date	Mar. 31, 2018
Product One [Member]
Derivative [Line Items]
Derivative, Type of Instrument	Swap
Derivative, Swap Type, Fixed Price | $ / MillionsofBTU-MMBTU	1.775
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	200
Product One [Member] | Contract Start Date [Member]
Derivative [Line Items]
Derivative, Inception Date	May 01, 2017
Product One [Member] | Contract End Date [Member]
Derivative [Line Items]
Derivative, Maturity Date	Oct. 31, 2017
Product Two [Member]
Derivative [Line Items]
Derivative, Type of Instrument	Swap
Derivative, Swap Type, Fixed Price | $ / MillionsofBTU-MMBTU	2.078
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	250
Product Two [Member] | Contract Start Date [Member]
Derivative [Line Items]
Derivative, Inception Date	Nov. 01, 2017
Product Two [Member] | Contract End Date [Member]
Derivative [Line Items]
Derivative, Maturity Date	Mar. 31, 2018
Product Three [Member]
Derivative [Line Items]
Derivative, Type of Instrument	Swap
Derivative, Swap Type, Fixed Price | $ / MillionsofBTU-MMBTU	2.536
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	500
Product Three [Member] | Contract Start Date [Member]
Derivative [Line Items]
Derivative, Inception Date	Dec. 01, 2017
Product Three [Member] | Contract End Date [Member]
Derivative [Line Items]
Derivative, Maturity Date	May 31, 2018