MISCELLANEOUS INCOME (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Income and Expenses [Abstract]
Equity portion of AFUDC	$ 12.5teg_EquityPortionOfAFUDC	$ 10.8teg_EquityPortionOfAFUDC	$ 2.9teg_EquityPortionOfAFUDC
Federal excise tax credit	4.4teg_OtherIncomeFederalExciseTaxCredits	4.1teg_OtherIncomeFederalExciseTaxCredits	0teg_OtherIncomeFederalExciseTaxCredits
Gain on sale of land at the holding company	3.5us-gaap_GainLossOnDispositionOfAssets1	0us-gaap_GainLossOnDispositionOfAssets1	0us-gaap_GainLossOnDispositionOfAssets1
Key executive life insurance income for retired employees	2.9	2.2	2.6
Gains on exchange-traded funds	2.9us-gaap_MarketableSecuritiesGainLoss	2.2us-gaap_MarketableSecuritiesGainLoss	1.3us-gaap_MarketableSecuritiesGainLoss
Other	4.8teg_OtherNonoperatingIncomeExpenseOther	2.6teg_OtherNonoperatingIncomeExpenseOther	2.2teg_OtherNonoperatingIncomeExpenseOther
Total miscellaneous income	$ 31.0us-gaap_OtherNonoperatingIncomeExpense	$ 21.9us-gaap_OtherNonoperatingIncomeExpense	$ 9.0us-gaap_OtherNonoperatingIncomeExpense

Represents income related to the equity portion of AFUDC (allowance for funds used during construction).

No definition available.

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- Definition

Represents the income associated with key executive life insurance for retired employees.

+ References

No definition available.

+ Details

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- Definition

Represents income associated with federal excise tax credits.

+ References

No definition available.

+ Details

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- Definition

The net amount of other income and expense amounts, the components of which are not separately stated in the disclosure for other nonoperating income and expenses.

+ References

No definition available.

+ Details

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- Definition

Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2443-110228

+ Details

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- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(c))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(c)-(e)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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- Details

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- Definition

The net amount of other income and expense amounts, the components of which are not separately disclosed on the income statement, resulting from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business) also known as other nonoperating income (expense) recognized for the period. Such amounts may include: (a) dividends, (b) interest on securities, (c) net gains or losses on securities, (d) unusual costs, (e) gains or losses on foreign exchange transactions, and (f) miscellaneous other income and expense items.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 9

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.9)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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