COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS AND PURCHASE ORDER COMMITMENTS (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 2,421.3
2014	418.8
2015	534.2
2016	293.8
2017	216.1
2018	155.3
Later Years	803.1
Purchase orders	1,173.0
Natural gas utility supply and transportation | Regulated operations | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	772.5
2014	91.7
2015	171.3
2016	161.8
2017	127.0
2018	76.2
Later Years	144.5
Purchased power | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	907.2
2014	42.1
2015	54.7
2016	42.9
2017	53.5
2018	56.5
Later Years	657.5
Coal supply and transportation | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	127.9
2014	26.1
2015	42.8
2016	18.5
2017	20.6
2018	19.9
Later Years	0
Nonregulated electricity and natural gas supply | Nonutility and nonregulated operations | Integrys Energy Services
Minimum future commitments for purchase obligations
Total Amounts Committed	613.7
2014	258.9
2015	265.4
2016	70.6
2017	15.0
2018	2.7
Later Years	1.1
Purchase orders	46.7
UPPCO | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Purchase orders	13.1
UPPCO | Purchased power | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 14.3