Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Swaps - Natural Gas	Mar. 31, 2019 MMBTU / d $ / MMBTU
November-December 2019 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged | MMBTU / d	20,000
Weighted average price of hedge | $ / MMBTU	3.20
2019 Commodity Contract
Derivative [Line Items]
Volume Hedged | MMBTU / d	1,246,873
Weighted average price of hedge | $ / MMBTU	2.80
2019 Commodity Contract | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Volume Hedged | MMBTU / d	150,000
Weighted average price of hedge | $ / MMBTU	2.81
2020 Commodity Contract
Derivative [Line Items]
Volume Hedged | MMBTU / d	190,000
Weighted average price of hedge | $ / MMBTU	2.75
2020 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged | MMBTU / d	140,000
Weighted average price of hedge | $ / MMBTU	2.78
January-March 2020 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged | MMBTU / d	20,000
Weighted average price of hedge | $ / MMBTU	3.20