Derivatives Hedges and Financial Instruments - Additional Information (Detail) $ / shares in Units, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) MMBTU $ / shares $ / MMBTU	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) MMBTU $ / shares $ / MMBTU shares	Jun. 30, 2020 USD ($)	Dec. 31, 2020 MMBTU $ / shares $ / MMBTU shares
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Gain (loss) on sale of natural gas | $		$ 0.7	$ 2.7	$ (0.7)
Realized gain on sale of natural gas | $			1.5
Non-Operating Other Income and Expense [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on fair value of embedded derivative | $	$ (0.7)	$ 0.1	$ (1.2)	$ 0.8
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	$ 6.05		$ 6.05		$ 3.39
Natural Gas Purchase Commitments
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Natural gas purchase commitments volume | MMBTU	4.7		4.7
Weighted average cost of natural gas per unit	2.75		2.75
Weighted average natural gas market value per unit	3.40		3.40
Natural Gas Purchase Commitments | 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