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Fair Value Measurements - Recurring Narrative (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)
Commodity derivatives | Level 3 | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward commodity price	$ 0.26
Commodity derivatives | Level 3 | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward commodity price	$ 1.47
Embedded derivatives in commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative renewal term	5 years
Embedded derivatives in commodity contracts | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Probability of renewal first term	62.50%
Probability of renewal second term	82.00%

X

- Definition

Represents the term for which the counterparty has the option to renew a purchase agreement related to an embedded derivative.

+ References

No definition available.

+ Details

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X

- Definition

Represents the forward commodity price used as an input to measure fair value.

+ References

No definition available.

+ Details

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X

- Definition

Represents the percentage of likelihood of renewal of the commodity contract with embedded derivative.

+ References

No definition available.

+ Details

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X

- Definition

Represents the percentage of likelihood of renewal of the commodity contract with embedded derivative for the second term of the contract.

+ References

No definition available.

+ Details

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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