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Derivative Instruments - Effects of Gains and Losses from Derivative Instruments (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Energy Marketing Contract Liabilities, Current |  | $ 14,871 | $ 6,247 |
Gain (Loss) on derivatives |  | (7,703) | (3,195) | $ (45)
Revenue | Derivatives not designated as hedging instruments | Forward Contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | [1] | 0 | 112 | (546)
Cost of Sales | Derivatives not designated as hedging instruments | Propane Swap Agreement |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative, Gain (Loss) on Derivative, Net |  | (13) | 8 | 7
Cost of Sales | Derivatives not designated as hedging instruments | Future [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gain (Loss) on derivatives |  | (3,189) | (3,633) | (541)
Cost of Sales | Derivatives designated as fair value hedges | Put Or Call Option [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative, Gain (Loss) on Derivative, Net |  | 0 | (9) | 49
Cost of Sales | Derivatives designated as fair value hedges | Propane Swap Agreement |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative, Gain (Loss) on Derivative, Net |  | (647) | 1,607 | (364)
Cost of Sales | Derivatives designated as fair value hedges | Future [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gain (Loss) on derivatives |  |  |  | 345
Propane Inventory | Derivatives designated as fair value hedges | Future [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gain (Loss) on derivatives |  | 0 | 0 | (233)
Other Comprehensive Income (Loss) | Derivatives designated as fair value hedges | Natural Gas Futures [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gain (Loss) on derivatives |  | 532 | (1,476) | 222
Other Comprehensive Income (Loss) | Derivatives designated as fair value hedges | Propane Swap Agreement |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gain (Loss) on derivatives |  | (2,773) | 487 | $ 1,016
Mark To Market Energy Liabilities [Member] | Derivatives not designated as hedging instruments | Natural Gas Futures [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Energy Marketing Contract Liabilities, Current |  | 4,562 | 5,776 |
Mark To Market Energy Liabilities [Member] | Derivatives designated as fair value hedges | Propane Swap Agreement |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Energy Marketing Contract Liabilities, Current |  | 0 | 469 |
2017 [Member] | Mark To Market Energy Liabilities [Member] | Derivatives designated as fair value hedges | Propane Swap Agreement |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Energy Marketing Contract Liabilities, Current |  | $ 1,604 | $ 2 |
[1] | All of the realized and unrealized gain (loss) on forward contracts represents the effect of trading activities on our consolidated statements of income.

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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- 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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- Definition Current liabilities attributable to energy marketing activities (trading activities). Current energy marketing (trading activities) are when entities engage in both financial and physical trading to increase profits, manage their commodity price risk and enhance system reliability. They may trade electricity, coal, natural gas and oil. These entities typically use a variety of financial instruments, including forward contracts, options and swaps. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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