GOODWILL AND OTHER INTANGIBLE ASSETS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
GOODWILL AND OTHER INTANGIBLE ASSETS
Changes in the carrying amount of goodwill	$ 0	$ 0
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	64.2		46.6
Accumulated amortization of intangible assets	(18.9)		(17.9)
Amortized intangible assets, net	45.3		28.7
Unamortized intangible assets
Total intangible assets, gross	74.4		56.8
Total intangible assets	55.5		38.9
Nonregulated cost of sales
Amortized intangible assets
Amortization expense	0.4	1.6
Amortization expense for the next five fiscal years
Amortization expense, 2013	6.7
Amortization expense, 2014	1.2
Amortization expense, 2015	1.1
Amortization expense, 2016	0.9
Amortization expense, 2017	0.9
Depreciation and amortization
Amortized intangible assets
Amortization expense	0.5	0.7
Amortization expense for the next five fiscal years
Amortization expense, 2013	3.8
Amortization expense, 2014	4.1
Amortization expense, 2015	4.1
Amortization expense, 2016	3.9
Amortization expense, 2017	3.8
Customer-related
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	22.4		22.4
Accumulated amortization of intangible assets	(15.0)		(14.7)
Amortized intangible assets, net	7.4		7.7
Amortization period	P9Y
Contractual service agreements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	15.6
Amortized intangible assets, net	15.6
Amortization period	P7Y
Contractual service agreements | Fox Energy Company, LLC | WPS
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	15.6
Amortization period	P7Y
Patents/intellectual property
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	7.2		7.2
Accumulated amortization of intangible assets	(0.3)		(0.3)
Amortized intangible assets, net	6.9		6.9
Amortization period	P9Y
Compressed natural gas fueling contract assets
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	5.6		5.6
Accumulated amortization of intangible assets	(1.7)		(1.3)
Amortized intangible assets, net	3.9		4.3
Amortization period	P8Y
Renewable energy credits
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	5.1		3.1
Amortized intangible assets, net	5.1		3.1
Nonregulated easements
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	3.8		3.8
Accumulated amortization of intangible assets	(1.0)		(0.9)
Amortized intangible assets, net	2.8		2.9
Amortization period	P11Y
Customer-owned equipment modifications
Amortized intangible assets
Amortized intangible assets, carrying amount, gross	4.0		4.0
Accumulated amortization of intangible assets	(0.6)		(0.5)
Amortized intangible assets, net	3.4		3.5
Amortization period	P11Y
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 CNG Systems
Unamortized intangible assets
Unamortized intangible asset, carrying amount	$ 1.5		$ 1.5