FAIR VALUE - LEVEL 3 RECONCILIATION (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	$ 19.4	$ 5.7	$ 14.3	$ (4.9)
Net realized and unrealized gains (losses) included in earnings	10.0	7.4	16.0	10.6
Net unrealized gains (losses) recorded as regulatory assets or liabilities	(0.6)	(3.9)	2.6	1.9
Purchases	1.0	0.7	7.8	6.5
Sales	(1.0)		(1.7)	(0.1)
Settlements	(4.3)	(4.3)	(13.5)	(10.7)
Net transfers into Level 3	0.8	2.3	8.8	10.0
Net transfers out of Level 3	(2.9)	0.5	(11.9)	(4.9)
Balance at the end of the period	22.4	8.4	22.4	8.4
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	7.4	6.1	13.0	8.9
Integrys Energy Services | Contingent consideration
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	(6.6)	(7.7)	(7.8)	0
Net realized and unrealized gains (losses) included in earnings	2.3	0	2.3	0
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	0	0	0
Purchases	0	0	0	(7.7)
Sales	0		0	0
Settlements	4.3	0	5.5	0
Net transfers into Level 3	0	0	0	0
Net transfers out of Level 3	0	0	0	0
Balance at the end of the period	0	(7.7)	0	(7.7)
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	0	0	0	0
Integrys Energy Services | Natural gas contracts
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	5.0	7.7	10.4	3.9
Net realized and unrealized gains (losses) included in earnings	6.2	4.2	0.2	1.3
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	0	0	0
Purchases	0	0	0	7.0
Sales	0		0	0
Settlements	(1.0)	(2.5)	(1.5)	(4.3)
Net transfers into Level 3	0.7	2.5	2.3	4.0
Net transfers out of Level 3	0	(0.3)	(0.5)	(0.3)
Balance at the end of the period	10.9	11.6	10.9	11.6
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	6.2	4.2	0.2	1.3
Integrys Energy Services | Electric contracts
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	14.9	4.1	12.4	(4.3)
Net realized and unrealized gains (losses) included in earnings	1.2	1.9	12.8	7.6
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	0	0	0
Purchases	0.9	0.7	2.2	2.3
Sales	0		(0.7)	0
Settlements	(5.7)	(4.6)	(13.3)	(4.3)
Net transfers into Level 3	0.1	(0.2)	6.5	6.0
Net transfers out of Level 3	(2.9)	0.8	(11.4)	(4.6)
Balance at the end of the period	8.5	2.7	8.5	2.7
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	1.2	1.9	12.8	7.6
Utility segments
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Impact on earnings of unrealized gains (losses) on level 3 instruments	0	0	0	0
Utility segments | FTRs
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	5.2	3.9	1.8	2.0
Net realized and unrealized gains (losses) included in earnings	0.3	1.3	0.7	1.7
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0.4	0.6	0.6	(0.3)
Purchases	0.1	0	5.6	4.9
Sales	(1.0)		(1.0)	(0.1)
Settlements	(2.0)	(2.8)	(4.7)	(5.2)
Net transfers into Level 3	0	0	0	0
Net transfers out of Level 3	0	0	0	0
Balance at the end of the period	3.0	3.0	3.0	3.0
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	0	0	0	0
Utility segments | Coal contracts
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	0.9	(2.3)	(2.5)	(6.5)
Net realized and unrealized gains (losses) included in earnings	0	0	0	0
Net unrealized gains (losses) recorded as regulatory assets or liabilities	(1.0)	(4.5)	2.0	2.2
Purchases	0	0	0	0
Sales	0		0	0
Settlements	0.1	5.6	0.5	3.1
Net transfers into Level 3	0	0	0	0
Net transfers out of Level 3	0	0	0	0
Balance at the end of the period	0	(1.2)	0	(1.2)
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	$ 0	$ 0	$ 0	$ 0