COMMITMENTS AND CONTINGENCIES - UNCONDITIONAL PURCHASE OBLIGATIONS AND PURCHASE ORDER COMMITMENTS (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 2,407.2
2014	611.4
2015	408.8
2016	263.1
2017	191.1
2018	130.6
Later Years	802.2
Purchase orders	1,163.3
Natural gas utility supply and transportation | Regulated operations | Natural Gas Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	793.7
2014	128.2
2015	169.7
2016	158.5
2017	122.6
2018	71.7
Later Years	143.0
Purchased power | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	918.0
2014	59.8
2015	49.4
2016	42.8
2017	53.2
2018	54.0
Later Years	658.8
Coal supply and transportation | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	94.6
2014	47.0
2015	28.5
2016	9.9
2017	5.9
2018	3.3
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	600.9
2014	376.4
2015	161.2
2016	51.9
2017	9.4
2018	1.6
Later Years	0.4
UPPCO | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Purchase orders	12.2
UPPCO | Purchased power | Regulated operations | Electric Utility
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 8.2