FAIR VALUE (Details 2) (Nonregulated Segments, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Natural gas contracts
Transfers between the levels of the fair value hierarchy
Transfers into Level 1 from Level 3	$ 0	$ 0	$ 0	$ 0
Transfers into Level 2 from Level 3	0	0.1	0	1.4
Transfers into Level 3 from Level 2	1.3	0.4	1.5	2.8
Transfers into Level 1 from Level 2	0	0	0	0
Transfers into Level 2 from Level 1	0	0	0	0
Transfers into Level 3 from Level 1	0	0	0	0
Electric contracts
Transfers between the levels of the fair value hierarchy
Transfers into Level 1 from Level 3	0	0	0	0
Transfers into Level 2 from Level 3	(0.1)	(3.8)	5.4	(3.9)
Transfers into Level 3 from Level 2	6.2	(3.8)	6.2	(8.8)
Transfers into Level 1 from Level 2	0	0	0	0
Transfers into Level 2 from Level 1	0	0	0	0
Transfers into Level 3 from Level 1	$ 0	$ 0	$ 0	$ 0

Name:	teg_FairValueAssetsAndLiabilitiesTransfersBetweenTheLevelsOfTheFairValueHierarchyLineItems
Namespace Prefix:	teg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 3.

+ References

No definition available.

+ Details

Name:	teg_FairValueLevel1ToLevel3TransfersAmount
Namespace Prefix:	teg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 3.

+ References

No definition available.

+ Details

Name:	teg_FairValueLevel2ToLevel3TransfersAmount
Namespace Prefix:	teg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 1.

+ References

No definition available.

+ Details

Name:	teg_FairValueLevel3ToLevel1TransfersAmount
Namespace Prefix:	teg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 2.

+ References

No definition available.

+ Details

Name:	teg_FairValueLevel3ToLevel2TransfersAmount
Namespace Prefix:	teg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ 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 (bb)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

+ Details

Name:	us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant