COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Coal supply and transportation | Electric Utility
Purchase and sales commitments
Long-term purchase and sale commitments	$ 186.3
Purchased power | Electric Utility
Purchase and sales commitments
Long-term purchase and sale commitments	1,213.9
Other commodities | Electric Utility
Purchase and sales commitments
Long-term purchase and sale commitments	5.4
Natural gas supply and transportation | Natural Gas Utility
Purchase and sales commitments
Long-term purchase and sale commitments	836.7
Electricity and natural gas supply | Integrys Energy Services
Purchase and sales commitments
Long-term purchase and sale commitments	218.9
Purchase orders
Purchase and sales commitments
Supply contract obligation amount	$ 536.1