WEC MERGER (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Business Combinations [Abstract]
Number of Wisconsin Energy shares per Integrys Energy Group share	1.128
Cash per share of Integrys Energy Group common stock	$ 18.58
Purchase price	$ 5,600.0
After-tax merger costs		$ 38.4	$ 4.1	$ 39.0	$ 4.1
After-tax merger cost for stock-based compensation and change-in-control		26.6		26.6
Receivables from WEC	27.3	27.3		27.3		$ 0.0
Payables to WEC	$ 1.8	$ 1.8		$ 1.8		$ 0.0
ICC ordered base rate freeze duration for PGL and NSG	2 years