FAIR VALUE - TRANSFERS BETWEEN LEVELS (Details) (Fair value measurements on a recurring basis, Integrys Energy Services, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Natural gas contracts
Transfers between the levels of the fair value hierarchy
Transfers into Level 2 from Level 1	$ 0	$ 0	$ 0	$ 0
Transfers into Level 3 from Level 1	0	0	0	0
Transfers into Level 1 from Level 2	0	0	0.1	0
Transfers into Level 3 from Level 2	0.7	2.5	2.3	4.0
Transfers into Level 1 from Level 3	0	0	0	0
Transfers into Level 2 from Level 3	0	0.3	0.5	0.3
Electric contracts
Transfers between the levels of the fair value hierarchy
Transfers into Level 2 from Level 1	0	0	0	0
Transfers into Level 3 from Level 1	0	(0.2)	0	(0.2)
Transfers into Level 1 from Level 2	1.0	0	1.2	0
Transfers into Level 3 from Level 2	0.1	0	6.5	6.2
Transfers into Level 1 from Level 3	0	0	0	0
Transfers into Level 2 from Level 3	$ 2.9	$ (0.8)	$ 11.4	$ 4.6