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FAIR VALUE MEASUREMENTS (Tables) | 6 Months Ended
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Jun. 30, 2018
Fair Value Disclosures [Abstract] |
Fair value of assets and liabilities measured on a recurring basis, categorized by level within the fair value hierarchy | The following tables summarize our financial assets and liabilities that were accounted for at fair value on a recurring basis, categorized by level within the fair value hierarchy:
|  | June 30, 2018
(in millions) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Derivative assets |  |  |  |  |  |  |  |
Natural gas contracts |  | $ | 5.4 |  |  | $ | 0.6 |  |  | $ | — |  |  | $ | 6.0 |
Petroleum products contracts |  | 0.4 |  |  | — |  |  | — |  |  | 0.4 |
FTRs |  | — |  |  | — |  |  | 16.7 |  |  | 16.7 |
Coal contracts |  | — |  |  | 0.6 |  |  | — |  |  | 0.6 |
Total derivative assets |  | $ | 5.8 |  |  | $ | 1.2 |  |  | $ | 16.7 |  |  | $ | 23.7 |
Investments held in rabbi trust |  | $ | 107.6 |  |  | $ | — |  |  | $ | — |  |  | $ | 107.6 |
Derivative liabilities |  |  |  |  |  |  |  |
Natural gas contracts |  | $ | 2.1 |  |  | $ | 0.2 |  |  | $ | — |  |  | $ | 2.3 |
Coal contracts |  | — |  |  | 0.3 |  |  | — |  |  | 0.3 |
Total derivative liabilities |  | $ | 2.1 |  |  | $ | 0.5 |  |  | $ | — |  |  | $ | 2.6 |
|  | December 31, 2017
(in millions) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Derivative assets |  |  |  |  |  |  |  |
Natural gas contracts |  | $ | 1.8 |  |  | $ | 3.9 |  |  | $ | — |  |  | $ | 5.7 |
Petroleum products contracts |  | 1.2 |  |  | — |  |  | — |  |  | 1.2 |
FTRs |  | — |  |  | — |  |  | 4.4 |  |  | 4.4 |
Coal contracts |  | — |  |  | 1.1 |  |  | — |  |  | 1.1 |
Total derivative assets |  | $ | 3.0 |  |  | $ | 5.0 |  |  | $ | 4.4 |  |  | $ | 12.4 |
Investments held in rabbi trust |  | $ | 120.7 |  |  | $ | — |  |  | $ | — |  |  | $ | 120.7 |
Derivative liabilities |  |  |  |  |  |  |  |
Natural gas contracts |  | $ | 7.0 |  |  | $ | 3.8 |  |  | $ | — |  |  | $ | 10.8 |
Coal contracts |  | — |  |  | 0.8 |  |  | — |  |  | 0.8 |
Total derivative liabilities |  | $ | 7.0 |  |  | $ | 4.6 |  |  | $ | — |  |  | $ | 11.6 |
Reconciliation of changes in fair value of items categorized as level 3 measurements | The following table summarizes the changes to derivatives classified as Level 3 in the fair value hierarchy:
|  | Three Months Ended June 30 |  | Six Months Ended June 30
(in millions) |  | 2018 |  | 2017 |  | 2018 |  | 2017
Balance at the beginning of the period |  | $ | 1.5 |  |  | $ | 1.7 |  |  | $ | 4.4 |  |  | $ | 5.1 |
Purchases |  | 18.4 |  |  | 13.8 |  |  | 18.4 |  |  | 13.8 |
Settlements |  | (3.2 | ) |  | (3.7 | ) |  | (6.1 | ) |  | (7.1 | )
Balance at the end of the period |  | $ | 16.7 |  |  | $ | 11.8 |  |  | $ | 16.7 |  |  | $ | 11.8 |
Schedule of carrying value and estimated fair value of financial instruments not recorded at fair value | The following table shows the financial instruments included on our balance sheets that are not recorded at fair value:
|  | June 30, 2018 |  | December 31, 2017
(in millions) |  | Carrying Amount |  | Fair Value |  | Carrying Amount |  | Fair Value
Preferred stock |  | $ | 30.4 |  |  | $ | 28.7 |  |  | $ | 30.4 |  |  | $ | 30.5 |
Long-term debt, including current portion * |  | 9,477.6 |  |  | 9,827.1 |  |  | 9,561.7 |  |  | 10,341.9 |
* | The carrying amount of long-term debt excludes capital lease obligations of $25.2 million and $27.0 million at June 30, 2018 and December 31, 2017, respectively.

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13467-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=75026489&loc=d3e13220-108610 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 + Details
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- Definition Tabular disclosure of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) using recurring unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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