GOODWILL AND OTHER INTANGIBLE ASSETS (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended					12 Months Ended		12 Months Ended												12 Months Ended					12 Months Ended									12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Trillium and Pinnacle	Sep. 30, 2011 Trillium and Pinnacle	Dec. 31, 2011 Customer-related y	Dec. 31, 2010 Customer-related	Dec. 31, 2011 Customer-related Trillium and Pinnacle y	Dec. 31, 2011 Contracts	Dec. 31, 2010 Contracts	Dec. 31, 2009 Contracts	Dec. 31, 2011 Natural gas and electric contract assets y	Dec. 31, 2010 Natural gas and electric contract assets	Dec. 31, 2010 Natural gas and electric contract liabilities	Dec. 31, 2011 Renewable energy credits	Dec. 31, 2010 Renewable energy credits	Dec. 31, 2011 Emission allowances	Dec. 31, 2010 Emission allowances	Dec. 31, 2011 Patents	Dec. 31, 2011 Patents Trillium and Pinnacle y	Dec. 31, 2011 Compressed natural gas fueling contract assets y	Dec. 31, 2011 Compressed natural gas fueling contract assets Trillium and Pinnacle y	Dec. 31, 2011 Nonregulated easements y	Dec. 31, 2010 Nonregulated easements	Dec. 31, 2011 Customer-owned equipment modifications y	Dec. 31, 2010 Customer-owned equipment modifications	Dec. 31, 2011 Other amortized intangible assets	Dec. 31, 2010 Other amortized intangible assets	Dec. 31, 2011 Trade name MGU	Dec. 31, 2010 Trade name MGU	Dec. 31, 2011 Trade name Trillium and Pinnacle	Dec. 31, 2011 Trade name Trillium	Dec. 31, 2011 Trade name Pinnacle CNG Systems	Dec. 31, 2011 Software Trillium and Pinnacle y
Intangible assets other than goodwill
Goodwill					$ 15.9
Total Amount Assigned				20.2	20.2			1.9												7.2		5.6									5.0			0.5
Amortization Period (in years)								15												18		10												3
Amortized intangible assets
Amortized intangible assets, carrying amount, gross						34.5	32.6					7.8	57.1		2.8	2.5	1.7	1.9	7.2		5.6		3.8	3.8	3.6	1.6	1.4	0.8
Accumulated amortization of intangible assets						(24.8)	(21.8)					(6.6)	(55.0)				(0.2)	(0.2)			(0.3)		(0.7)	(0.4)	(0.2)	(0.1)	(0.3)	(0.3)
Amortized intangible assets, net						9.7	10.8					1.2	2.1		2.8	2.5	1.5	1.7	7.2		5.3		3.1	3.4	3.4	1.5	1.1	0.5
Amortized intangible liabilities, carrying amount, gross														(10.5)
Accumulated amortization of intangible liabilities														10.5
Amortized intangible assets (liabilities) carrying amount, gross	68.4	89.8
Accumulated amortization of intangible assets (liabilities)	(33.1)	(67.3)
Amortized intangible assets (liabilities), net	35.3	22.5
Unamortized intangible asset, carrying amount																													5.2	5.2		3.5	1.5
Total intangible assets, gross	78.6	95.0
Accumulated amortization of total intangible assets	(33.1)	(67.3)
Total intangible assets	45.5	27.7
Weighted-average amortization period (in years)						10						3									9		12		12
Short-term intangible assets												0.5	0.9
Long-term intangible assets												0.7	1.2
Amortization expense	3.4	3.9	6.3						1.4	4.9	8.9
Amortization expense for the next five fiscal years
Amortization expense, 2012	2.7								4.8
Amortization expense, 2013	2.1								2.2
Amortization expense, 2014	1.8								1.7
Amortization expense, 2015	1.6								1.3
Amortization expense, 2016	$ 1.5								$ 1.0