STOCK-BASED COMPENSATION – STOCK-BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Information related to share based awards
Total stock-based compensation expense	$ 8.2	$ 12.5	$ 12.4	$ 16.0
Deferred income tax benefit	3.3	5.0	5.0	6.4
Awards that were vested due to WEC merger
Information related to share based awards
Total stock-based compensation expense	22.7		22.7
Stock options
Information related to share based awards
Total stock-based compensation expense	0.0	0.5	0.0	0.8
Performance stock rights
Information related to share based awards
Total stock-based compensation expense	2.7	9.2	3.3	9.7
Restricted share units
Information related to share based awards
Total stock-based compensation expense	4.8	2.6	8.2	5.1
Nonemployee director deferred stock units
Information related to share based awards
Total stock-based compensation expense	$ 0.7	$ 0.2	$ 0.9	$ 0.4