GOODWILL AND OTHER INTANGIBLE ASSETS (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	$ 71.9	$ 46.6
Accumulated amortization of intangible assets	(23.5)	(17.9)
Amortized intangible assets, net	48.4	28.7
Unamortized intangible assets
Total intangible assets, gross	82.1	56.8
Total intangible assets, net	58.6	38.9
Amortization expense for the next five fiscal years
Future amortization expense period	5 years
Nonregulated cost of sales
Amortized intangible assets
Amortization expense	2.1	2.5	1.3
Amortization expense for the next five fiscal years
Amortization expense, 2014	3.4
Amortization expense, 2015	2.0
Amortization expense, 2016	1.1
Amortization expense, 2017	0.9
Amortization expense, 2018	0.8
Depreciation and amortization
Amortized intangible assets
Amortization expense	4.2	2.5	3.4
Amortization expense for the next five fiscal years
Amortization expense, 2014	4.3
Amortization expense, 2015	4.2
Amortization expense, 2016	4.0
Amortization expense, 2017	3.9
Amortization expense, 2018	3.8
Customer-related
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	26.8	22.4
Accumulated amortization of intangible assets	(15.7)	(14.7)
Amortized intangible assets, net	11.1	7.7
Weighted-average amortization period	P11Y
Contractual service agreements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	15.6	0
Accumulated amortization of intangible assets	(1.8)	0
Amortized intangible assets, net	13.8	0
Weighted-average amortization period	P6Y
Renewable energy credits
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	8.4	3.1
Accumulated amortization of intangible assets	0	0
Amortized intangible assets, net	8.4	3.1
Compressed natural gas fueling contract assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	5.6	5.6
Accumulated amortization of intangible assets	(2.7)	(1.3)
Amortized intangible assets, net	2.9	4.3
Weighted-average amortization period	P7Y
Customer-owned equipment modifications
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	4.0	4.0
Accumulated amortization of intangible assets	(0.9)	(0.5)
Amortized intangible assets, net	3.1	3.5
Weighted-average amortization period	P10Y
Natural gas and electric contract assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.9	0
Accumulated amortization of intangible assets	(0.5)	0
Amortized intangible assets, net	3.4	0
Weighted-average amortization period	P4Y
Nonregulated easements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.7	3.8
Accumulated amortization of intangible assets	(1.1)	(0.9)
Amortized intangible assets, net	2.6	2.9
Weighted-average amortization period	P10Y
Patents/intellectual property
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.4	7.2
Accumulated amortization of intangible assets	(0.5)	(0.3)
Amortized intangible assets, net	2.9	6.9
Weighted-average amortization period	P9Y
Other amortized intangible assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	0.5	0.5
Accumulated amortization of intangible assets	(0.3)	(0.2)
Amortized intangible assets, net	0.2	0.3
Trade name | MGU
Unamortized intangible assets
Unamortized intangible asset, carrying amount	5.2	5.2
Trade name | Trillium
Unamortized intangible assets
Unamortized intangible asset, carrying amount	3.5	3.5
Trade name | Pinnacle
Unamortized intangible assets
Unamortized intangible asset, carrying amount	$ 1.5	$ 1.5