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Inventories and natural gas in storage | 3 Months Ended
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Mar. 31, 2016
Inventory Disclosure [Abstract] |
Inventories and natural gas in storage | Inventories and natural gas in storage Natural gas in storage for the Company's regulated operations is generally carried at average cost, or cost using the last-in, first-out method. Crude oil and refined products at Dakota Prairie Refinery are carried at lower of cost or market value using the last-in, first-out method. All other inventories are stated at the lower of average cost or market value. The portion of the cost of natural gas in storage expected to be used within one year is included in inventories. Inventories consisted of:
| March 31, 2016 |  | March 31, 2015 |  | December 31, 2015 |
| (In thousands)
Aggregates held for resale | $ | 127,101 |  | $ | 112,029 |  | $ | 115,854 |
Asphalt oil | 52,065 |  | 89,578 |  | 36,498 |
Natural gas in storage (current) | 11,305 |  | 9,303 |  | 21,023 |
Materials and supplies | 21,645 |  | 57,073 |  | 16,997 |
Merchandise for resale | 17,441 |  | 15,688 |  | 15,318 |
Crude oil | 3,034 |  | 7,076 |  | 4,678 |
Refined products | 14,022 |  | — |  | 8,498 |
Other | 30,199 |  | 37,325 |  | 34,861 |
Total | $ | 276,812 |  | $ | 328,072 |  | $ | 253,727 |
The remainder of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, is included in other assets and was $49.1 million, $49.3 million and $49.1 million at March 31, 2016 and 2015, and December 31, 2015, respectively.

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- Definition The entire disclosure for inventory. Includes, but is not limited to, the basis of stating inventory, the method of determining inventory cost, the classes of inventory, and the nature of the cost elements included in inventory. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766 + Details
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