Derivatives Hedges and Financial Instruments - Additional Information (Detail) $ / shares in Units, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MMBTU $ / shares $ / MMBTU shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Gain (loss) on sale of natural gas | $	$ (1.6)	$ 0.0	$ 0.0
Unrealized loss on sale of natural gas | $	1.2
Non-Operating Other Income and Expense [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on fair value of embedded derivative | $	$ 0.1	$ 0.5	$ 1.2
Embedded Derivative [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Participating right in dividends and liquidating distributions expressed in number of common shares | shares	303,646	303,646
Embedded Derivative [Member] | Common Stock [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Common stock per share | $ / shares	$ 3.39	$ 4.20
Natural Gas Purchase Commitments [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Natural gas purchase commitments volume | MMBTU	3.8
Weighted average cost of natural gas per unit	2.80
Weighted average natural gas market value per unit	2.36
Natural Gas Purchase Commitments [Member] | Level 2 [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Natural gas purchase commitments volume | MMBTU	7.3
Weighted average cost of natural gas per unit	2.65
Weighted average natural gas market value per unit	2.49