GOODWILL AND OTHER INTANGIBLE ASSETS – INTANGIBLE ASSETS OTHER THAN GOODWILL (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended								9 Months Ended		1 Months Ended	9 Months Ended						9 Months Ended		9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Regulatory assets	Sep. 30, 2014 Nonregulated cost of sales	Sep. 30, 2013 Nonregulated cost of sales	Sep. 30, 2014 Nonregulated cost of sales	Sep. 30, 2013 Nonregulated cost of sales	Sep. 30, 2014 Depreciation and amortization	Sep. 30, 2013 Depreciation and amortization	Sep. 30, 2014 Depreciation and amortization	Sep. 30, 2013 Depreciation and amortization	Sep. 30, 2014 IES's retail energy business Nonregulated cost of sales	Sep. 30, 2013 IES's retail energy business Nonregulated cost of sales	Sep. 30, 2014 IES's retail energy business Nonregulated cost of sales	Sep. 30, 2013 IES's retail energy business Nonregulated cost of sales	Sep. 30, 2014 IES's retail energy business Depreciation and amortization	Sep. 30, 2013 IES's retail energy business Depreciation and amortization	Sep. 30, 2014 IES's retail energy business Depreciation and amortization	Sep. 30, 2013 IES's retail energy business Depreciation and amortization	Sep. 30, 2014 Other Nonregulated cost of sales	Sep. 30, 2013 Other Nonregulated cost of sales	Sep. 30, 2014 Other Nonregulated cost of sales	Sep. 30, 2013 Other Nonregulated cost of sales	Sep. 30, 2014 Other Depreciation and amortization	Sep. 30, 2013 Other Depreciation and amortization	Sep. 30, 2014 Other Depreciation and amortization	Sep. 30, 2013 Other Depreciation and amortization	Sep. 30, 2014 Trade name MGU	Dec. 31, 2013 Trade name MGU	Sep. 30, 2014 Trade name Trillium	Dec. 31, 2013 Trade name Trillium	Sep. 30, 2014 Trade name Pinnacle	Dec. 31, 2013 Trade name Pinnacle	Sep. 30, 2014 Contractual service agreements	Dec. 31, 2013 Contractual service agreements	Oct. 31, 2014 Contractual service agreements Subsequent event	Sep. 30, 2014 Customer-related	Dec. 31, 2013 Customer-related	Sep. 30, 2014 Customer-related IES's retail energy business	Dec. 31, 2013 Customer-related IES's retail energy business	Sep. 30, 2014 Renewable energy credits	Dec. 31, 2013 Renewable energy credits	Sep. 30, 2014 Customer-owned equipment modifications	Dec. 31, 2013 Customer-owned equipment modifications	Sep. 30, 2014 Patents/intellectual property	Dec. 31, 2013 Patents/intellectual property	Sep. 30, 2014 Nonregulated easements	Dec. 31, 2013 Nonregulated easements	Sep. 30, 2014 Compressed natural gas fueling contract assets	Dec. 31, 2013 Compressed natural gas fueling contract assets	Sep. 30, 2014 Natural gas and electric contract assets	Dec. 31, 2013 Natural gas and electric contract assets	Sep. 30, 2014 Other amortized intangible assets	Dec. 31, 2013 Other amortized intangible assets
Intangible assets other than goodwill
Total intangible assets, gross	$ 81.2	$ 82.1
Total intangible assets, net	51.7	58.6
Amortized intangible assets
Amortized intangible assets, gross carrying amount	71.0	71.9																																15.6	15.6		26.8	26.8			7.4	8.4	4.0	4.0	3.4	3.4	3.9	3.7	5.6	5.6	3.8	3.9	0.5	0.5
Accumulated amortization of intangible assets	(29.5)	(23.5)																																(3.5)	(1.8)		(16.9)	(15.7)			0	0	(1.1)	(0.9)	(0.7)	(0.5)	(1.4)	(1.1)	(3.3)	(2.7)	(2.3)	(0.5)	(0.3)	(0.3)
Amortized intangible assets, net	41.5	48.4																																12.1	13.8		9.9	11.1	8.3	9.3	7.4	8.4	2.9	3.1	2.7	2.9	2.5	2.6	2.3	2.9	1.5	3.4	0.2	0.2
Contractual service agreements with shortened amortization period																																				1
Weighted-average amortization period																																		4 years			12 years						10 years		8 years		10 years		6 years
Recovery period																																		7 years
Amortization expense				0.6	0.6	2.7	1.5	1.1	1.3	3.3	3.0	0.3	0.2	1.8	0.3	0.3	0.5	1.0	1.3	0.3	0.4	0.9	1.2	0.8	0.8	2.3	1.7
Unamortized intangible assets
Unamortized intangible assets, 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			0.3	1.2		1.2		3.0		3.0
Amortization expense, 2015			1.0	1.1		1.1		3.0		3.0
Amortization expense, 2016			1.0	0.9		0.9		2.9		2.9
Amortization expense, 2017			0.5	0.9		0.9		2.4		2.4
Amortization expense, 2018			$ 0	$ 0.8		$ 0.8		$ 1.9		$ 1.9