GOODWILL AND OTHER INTANGIBLE ASSETS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the carrying amount of goodwill	$ 0	$ 0
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	73.1		71.9
Accumulated amortization of intangible assets	(25.6)		(23.5)
Amortized intangible assets, net	47.5		48.4
Unamortized intangible assets
Total intangible assets, gross	83.3		82.1
Total intangible assets, net	57.7		58.6
Amortization expense for the next five years
Future amortization expense period	5 years
Nonregulated cost of sales
Amortized intangible assets
Amortization expense	1.0	0.4
Amortization expense for the next five 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	1.1	0.5
Amortization expense for the next five 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
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
Customer-related
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	26.8		26.8
Accumulated amortization of intangible assets	(16.1)		(15.7)
Amortized intangible assets, net	10.7		11.1
Weighted-average amortization period	11 years
Contractual service agreements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	15.6		15.6
Accumulated amortization of intangible assets	(2.4)		(1.8)
Amortized intangible assets, net	13.2		13.8
Weighted-average amortization period	6 years
Renewable energy credits
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	9.6		8.4
Accumulated amortization of intangible assets	0		0
Amortized intangible assets, net	9.6		8.4
Compressed natural gas fueling contract assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	5.6		5.6
Accumulated amortization of intangible assets	(2.9)		(2.7)
Amortized intangible assets, net	2.7		2.9
Weighted-average amortization period	7 years
Customer-owned equipment modifications
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	4.0		4.0
Accumulated amortization of intangible assets	(0.9)		(0.9)
Amortized intangible assets, net	3.1		3.1
Weighted-average amortization period	10 years
Natural gas and electric contract assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.9		3.9
Accumulated amortization of intangible assets	(1.2)		(0.5)
Amortized intangible assets, net	2.7		3.4
Weighted-average amortization period	4 years
Nonregulated easements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.7		3.7
Accumulated amortization of intangible assets	(1.2)		(1.1)
Amortized intangible assets, net	2.5		2.6
Weighted-average amortization period	10 years
Patents/intellectual property
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.4		3.4
Accumulated amortization of intangible assets	(0.6)		(0.5)
Amortized intangible assets, net	2.8		2.9
Weighted-average amortization period	8 years
Other amortized intangible assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	0.5		0.5
Accumulated amortization of intangible assets	(0.3)		(0.3)
Amortized intangible assets, net	$ 0.2		$ 0.2