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Inventories and natural gas in storage | 6 Months Ended
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Jun. 30, 2015
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:
| June 30, 2015 | June 30, 2014 | December 31, 2014
| (In thousands)
Aggregates held for resale | $ | 123,457 |  | $ | 112,129 |  | $ | 108,161 |
Asphalt oil | 79,422 |  | 76,525 |  | 42,135 |
Materials and supplies | 22,594 |  | 58,089 |  | 54,282 |
Merchandise for resale | 16,140 |  | 25,507 |  | 24,420 |
Refined products | 16,065 |  | — |  | — |
Natural gas in storage (current) | 11,310 |  | 10,903 |  | 19,302 |
Crude oil | 8,101 |  | — |  | 5,045 |
Other | 37,316 |  | 35,420 |  | 36,065 |
Total | $ | 314,405 |  | $ | 318,573 |  | $ | 289,410 |
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 $49.3 million, $47.4 million and $49.3 million at June 30, 2015 and 2014, and December 31, 2014, respectively.

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- Definition The entire disclosure for inventory. This may include, but is not limited to, the basis of stating inventory, the method of determining inventory cost, the major classes of inventory, and the nature of the cost elements included in inventory. If inventory is stated above cost, accrued net losses on firm purchase commitments for inventory and losses resulting from valuing inventory at the lower-of-cost-or-market may also be included. For LIFO inventory, may disclose the amount and basis for determining the excess of replacement or current cost over stated LIFO value and the effects of a LIFO quantities liquidation that impacts net income. + 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph a, b, c -Article 5 + Details
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