GOODWILL AND OTHER INTANGIBLE ASSETS - INTANGIBLE ASSETS OTHER THAN GOODWILL (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2014
Intangible assets other than goodwill
Total intangible assets, gross	$ 45.1	$ 44.9
Total intangible assets, net	33.2	37.5
Amortized intangible assets
Amortized intangible assets, gross carrying amount	34.9	34.7
Accumulated amortization of intangible assets	(11.9)	(7.4)
Amortized intangible assets, net	23.0	27.3
Amortization expense for the next five years
Future amortization expense period	5 years
Regulatory assets
Amortized intangible assets
Amortization expense	0.3	0	0
Amortization expense for the next five years
Amortization expense, 2015	1.0
Amortization expense, 2016	1.0
Amortization expense, 2017	0.5
Amortization expense, 2018	0
Amortization expense, 2019	0
Cost of sales
Amortized intangible assets
Amortization expense	1.2	1.6	1.3
Amortization expense for the next five years
Amortization expense, 2015	1.1
Amortization expense, 2016	0.9
Amortization expense, 2017	0.9
Amortization expense, 2018	0.8
Amortization expense, 2019	0.6
Depreciation and amortization
Amortized intangible assets
Amortization expense	3.0	2.5	1.0
Amortization expense for the next five years
Amortization expense, 2015	3.0
Amortization expense, 2016	2.9
Amortization expense, 2017	2.4
Amortization expense, 2018	1.9
Amortization expense, 2019	1.9
Trade name | MGU
Unamortized intangible assets
Unamortized intangible assets, carrying amount	5.2	5.2
Trade name | Trillium
Unamortized intangible assets
Unamortized intangible assets, carrying amount	3.5	3.5
Trade name | Pinnacle
Unamortized intangible assets
Unamortized intangible assets, carrying amount	1.5	1.5
Contractual service agreements
Amortized intangible assets
Amortized intangible assets, gross carrying amount	15.6	15.6
Accumulated amortization of intangible assets	(4.3)	(1.8)
Amortized intangible assets, net	11.3	13.8
Contractual service agreements with shortened amortization period				1
Weighted-average amortization period	4 years
Recovery period	7 years
Customer-owned equipment modifications
Amortized intangible assets
Amortized intangible assets, gross carrying amount	4.0	4.0
Accumulated amortization of intangible assets	(1.2)	(0.9)
Amortized intangible assets, net	2.8	3.1
Weighted-average amortization period	9 years
Intellectual property
Amortized intangible assets
Amortized intangible assets, gross carrying amount	3.4	3.4
Accumulated amortization of intangible assets	(0.8)	(0.5)
Amortized intangible assets, net	2.6	2.9
Weighted-average amortization period	8 years
Nonregulated easements
Amortized intangible assets
Amortized intangible assets, gross carrying amount	3.9	3.7
Accumulated amortization of intangible assets	(1.4)	(1.1)
Amortized intangible assets, net	2.5	2.6
Weighted-average amortization period	9 years
Compressed natural gas fueling contract assets
Amortized intangible assets
Amortized intangible assets, gross carrying amount	5.6	5.6
Accumulated amortization of intangible assets	(3.6)	(2.7)
Amortized intangible assets, net	2.0	2.9
Weighted-average amortization period	6 years
Customer-related
Amortized intangible assets
Amortized intangible assets, gross carrying amount	1.9	1.9
Accumulated amortization of intangible assets	(0.3)	(0.1)
Amortized intangible assets, net	1.6	1.8
Weighted-average amortization period	12 years
Other amortized intangible assets
Amortized intangible assets
Amortized intangible assets, gross carrying amount	0.5	0.5
Accumulated amortization of intangible assets	(0.3)	(0.3)
Amortized intangible assets, net	$ 0.2	$ 0.2