OTHER INTANGIBLE ASSETS (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				9 Months Ended		9 Months Ended		9 Months Ended				9 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Nonregulated cost of sales	Sep. 30, 2012 Nonregulated cost of sales	Sep. 30, 2013 Nonregulated cost of sales	Sep. 30, 2012 Nonregulated cost of sales	Sep. 30, 2013 Depreciation and amortization	Sep. 30, 2012 Depreciation and amortization	Sep. 30, 2013 Depreciation and amortization	Sep. 30, 2012 Depreciation and amortization	Sep. 30, 2013 Customer-related	Dec. 31, 2012 Customer-related	Sep. 30, 2013 Contractual service agreements	Dec. 31, 2012 Contractual service agreements	Sep. 30, 2013 Patents/intellectual property	Dec. 31, 2012 Patents/intellectual property	Sep. 30, 2013 Compressed natural gas fueling contract assets	Dec. 31, 2012 Compressed natural gas fueling contract assets	Sep. 30, 2013 Renewable energy credits	Dec. 31, 2012 Renewable energy credits	Sep. 30, 2013 Nonregulated easements	Dec. 31, 2012 Nonregulated easements	Sep. 30, 2013 Customer-owned equipment modifications	Dec. 31, 2012 Customer-owned equipment modifications	Sep. 30, 2013 Other amortized intangible assets	Dec. 31, 2012 Other amortized intangible assets	Sep. 30, 2013 Trade name MGU	Dec. 31, 2012 Trade name MGU	Sep. 30, 2013 Trade name Trillium	Dec. 31, 2012 Trade name Trillium	Sep. 30, 2013 Trade name Pinnacle	Dec. 31, 2012 Trade name Pinnacle
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	$ 66.8	$ 46.6									$ 26.8	$ 22.4	$ 15.6	$ 0	$ 3.4	$ 7.2	$ 5.6	$ 5.6	$ 5.1	$ 3.1	$ 3.7	$ 3.8	$ 4.0	$ 4.0	$ 2.6	$ 0.5
Accumulated amortization of intangible assets	(21.9)	(17.9)									(15.3)	(14.7)	(1.2)	0	(0.4)	(0.3)	(2.3)	(1.3)	0	0	(1.1)	(0.9)	(0.8)	(0.5)	(0.8)	(0.2)
Amortized intangible assets, net	44.9	28.7									11.5	7.7	14.4	0	3.0	6.9	3.3	4.3	5.1	3.1	2.6	2.9	3.2	3.5	1.8	0.3
Weighted-average amortization period											P11Y		P7Y		P9Y		P7Y				P11Y		P11Y
Amortization expense			0.6	0.3	1.5	2.2	1.3	0.5	3.0	2.0
Unamortized intangible assets
Unamortized intangible asset, carrying amount																											5.2	5.2	3.5	3.5	1.5	1.5
Total intangible assets, gross	77.0	56.8
Total intangible assets, net	55.1	38.9
Amortization expense for the next five fiscal years
Amortization expense, 2013			7.2		7.2		4.2		4.2
Amortization expense, 2014			2.2		2.2		4.3		4.3
Amortization expense, 2015			1.4		1.4		4.2		4.2
Amortization expense, 2016			0.9		0.9		4.0		4.0
Amortization expense, 2017			$ 0.9		$ 0.9		$ 3.9		$ 3.9