ASSET RETIREMENT OBLIGATIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Changes in asset retirement obligations
Asset retirement obligations at the beginning of the period	$ 411.2	$ 397.2	$ 320.9
Accretion	20.9	20.4	17.1
Additions and revisions to estimated cash flows	70.6	(0.8)	64.9
Settlements	(11.2)	(5.6)	(5.7)
Asset retirement obligations at the end of the period	491.5	411.2	397.2
Utility segments
Changes in asset retirement obligations
Asset retirement obligations at the beginning of the period	409.0	396.7	320.9
Accretion	20.8	20.3	17.1
Additions and revisions to estimated cash flows	70.1	(2.4)	64.4
Settlements	(11.2)	(5.6)	(5.7)
Asset retirement obligations at the end of the period	488.7	409.0	396.7
Nonregulated segments | Integrys Energy Services
Changes in asset retirement obligations
Asset retirement obligations at the beginning of the period	2.2	0.5	0
Accretion	0.1	0.1	0
Additions and revisions to estimated cash flows	0.5	1.6	0.5
Settlements	0	0	0
Asset retirement obligations at the end of the period	$ 2.8	$ 2.2	$ 0.5