Fair Value Measurements (Recurring Narrative) (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)
Embedded derivative in commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative renewal term	5 years
Level 3 | Commodity derivative instruments | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward commodity price	$ 0.58
Level 3 | Commodity derivative instruments | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward commodity price	$ 1.01
Level 3 | Embedded derivative in commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Probability of renewal	90.00%
Probability of renewal second term	80.00%