Derivatives Hedges and Financial Instruments - Additional Information (Detail) $ / shares in Units, MMBTU in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) MMBTU $ / shares $ / MMBTU shares	Mar. 31, 2020 USD ($)	Dec. 31, 2020 MMBTU $ / shares $ / MMBTU shares
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Gain (loss) on sale of natural gas | $	$ 2.7	$ (0.7)
Realized gain on sale of natural gas | $	1.5	(0.2)
Non-Operating Other Income and Expense [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on fair value of embedded derivative | $	$ (0.4)	$ 0.6
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	$ 5.13		$ 3.39
Natural Gas Purchase Commitments [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Natural gas purchase commitments volume | MMBTU	9.0
Weighted average cost of natural gas per unit	2.69
Weighted average natural gas market value per unit	2.55
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