Derivative Financial Instruments and Hedging Activities - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2018 MMBTU $ / MMBTU	Dec. 31, 2017 USD ($) MMBTU $ / MMBTU	Dec. 31, 2016 USD ($) MMBTU $ / MMBTU	Dec. 31, 2015 USD ($)
Derivative Instruments And Hedging Activities [Line Items]
Cash flow hedges derivative instruments		$ 0
Gain on derivative financial instruments		16,800,000		$ 2,700,000
Loss on derivative financial instruments			$ 5,400,000
Cash settlements of derivative financial instruments		9,405,000	2,120,000	$ 1,230,000
Derivative asset at fair value		1,300,000
Derivative asset at carrying value		1,300,000
Derivative asset current at carrying value		$ 1,318,000
Derivative liability at fair value			6,000,000
Derivative liability at carrying value			6,000,000
Derivative liability current at carrying value			$ 6,030,000
Natural Gas Swap Agreements
Derivative Instruments And Hedging Activities [Line Items]
Commodity derivatives natural gas volumes hedged | MMBTU		2,565,000	23,400,000
Average Fixed NYMEX price | $ / MMBTU		3.38	3.37
Natural Gas Swap Agreements | Subsequent Event
Derivative Instruments And Hedging Activities [Line Items]
Commodity derivatives natural gas volumes hedged | MMBTU	21,600,000
Average Fixed NYMEX price | $ / MMBTU	3.00
Contract beginning dates	March and April 2018
Contract expiration dates	February and March 2019