EQUITY METHOD INVESTMENTS - INVESTMENTS (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Equity method investments
Equity method investments	$ 572.4	$ 540.9
ATC
Equity method investments
Equity method investments	536.7	508.4	476.6	439.4
INDU Solar Holdings, LLC
Equity method investments
Equity method investments	21.8	24.7	27.5	28.4
WRPC
Equity method investments
Equity method investments	7.7	7.0	7.3	7.7
AMP Trillium, LLC
Equity method investments
Equity method investments	5.5	0
Other
Equity method investments
Equity method investments	$ 0.7	$ 0.8