INTANGIBLE ASSETS OTHER THAN GOODWILL (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended								6 Months Ended		6 Months Ended				6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Nonregulated cost of sales	Jun. 30, 2013 Nonregulated cost of sales	Jun. 30, 2014 Nonregulated cost of sales	Jun. 30, 2013 Nonregulated cost of sales	Jun. 30, 2014 Depreciation and amortization	Jun. 30, 2013 Depreciation and amortization	Jun. 30, 2014 Depreciation and amortization	Jun. 30, 2013 Depreciation and amortization	Jun. 30, 2014 Trade name MGU	Dec. 31, 2013 Trade name MGU	Jun. 30, 2014 Trade name Trillium	Dec. 31, 2013 Trade name Trillium	Jun. 30, 2014 Trade name Pinnacle	Dec. 31, 2013 Trade name Pinnacle	Jun. 30, 2014 Contractual service agreements	Dec. 31, 2013 Contractual service agreements	Jun. 30, 2014 Customer-related	Dec. 31, 2013 Customer-related	Jun. 30, 2014 Renewable energy credits	Dec. 31, 2013 Renewable energy credits	Jun. 30, 2014 Customer-owned equipment modifications	Dec. 31, 2013 Customer-owned equipment modifications	Jun. 30, 2014 Patents/intellectual property	Dec. 31, 2013 Patents/intellectual property	Jun. 30, 2014 Compressed natural gas fueling contract assets	Dec. 31, 2013 Compressed natural gas fueling contract assets	Jun. 30, 2014 Nonregulated easements	Dec. 31, 2013 Nonregulated easements	Jun. 30, 2014 Natural gas and electric contract assets	Dec. 31, 2013 Natural gas and electric contract assets	Jun. 30, 2014 Other amortized intangible assets	Dec. 31, 2013 Other amortized intangible assets
Intangible assets other than goodwill
Total intangible assets, gross	$ 80.9	$ 82.1
Total intangible assets, net	53.1	58.6
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	70.7	71.9															15.6	15.6	26.8	26.8	7.2	8.4	4.0	4.0	3.4	3.4	5.6	5.6	3.7	3.7	3.9	3.9	0.5	0.5
Accumulated amortization of intangible assets	(27.8)	(23.5)															(3.0)	(1.8)	(16.5)	(15.7)	0	0	(1.0)	(0.9)	(0.6)	(0.5)	(3.1)	(2.7)	(1.3)	(1.1)	(2.0)	(0.5)	(0.3)	(0.3)
Amortized intangible assets, net	42.9	48.4															12.6	13.8	10.3	11.1	7.2	8.4	3.0	3.1	2.8	2.9	2.5	2.9	2.4	2.6	1.9	3.4	0.2	0.2
Weighted-average amortization period																	6 years		11 years				10 years		8 years		7 years		10 years		3 years
Amortization expense			1.1	0.5	2.1	0.9	1.1	1.2	2.2	1.7
Unamortized intangible assets
Unamortized intangible asset, carrying amount											5.2	5.2	3.5	3.5	1.5	1.5
Amortization expense for the next five years
Future amortization expense period	5 years
Amortization expense, 2014			3.4		3.4		4.3		4.3
Amortization expense, 2015			2.0		2.0		4.2		4.2
Amortization expense, 2016			1.1		1.1		4.0		4.0
Amortization expense, 2017			0.9		0.9		3.9		3.9
Amortization expense, 2018			$ 0.8		$ 0.8		$ 3.8		$ 3.8