Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting [Abstract]
Number of reporting segments	3								3
Segment Reporting Information [Line Items]
Revenues	$ 627.4	$ 539.4	$ 475.8	$ 682.6	$ 564.1	$ 422.7	$ 402.4	$ 740.3	$ 2,325.2	$ 2,129.5	$ 2,088.9
Net income (loss)	46.6	35.3	15.1	44.6	45.4	16.4	8.7	63.2	141.6	133.7	133.1
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									244.3	229.1	211.9
Interest Expense									106.5	104.6	100.0
Income Taxes									86.4	74.7	64.1
Capital Expenditures									321.3	277.2	432.0
Assets	4,878.9				4,764.2				4,878.9	4,764.2	4,671.8
Number of operating segments in the Nonutility Group	5								5
Portion of Indiana that is provided natural gas distribution and transportation services by the Gas Utility Services segment	67.00%								67.00%
Utility Group [Member]
Segment Reporting Information [Line Items]
Revenues									1,457.0	1,563.7	1,596.2
Net income (loss)									122.9	123.9	107.4
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									192.3	188.2	180.9
Interest Expense									80.3	81.4	79.2
Income Taxes									82.9	77.1	59.2
Capital Expenditures									233.4	225.1	302.7
Assets	3,974.5				3,924.5				3,974.5	3,924.5	3,823.1
Utility Group [Member] | Gas Utility Services [Member]
Segment Reporting Information [Line Items]
Revenues									819.1	954.1	1,066.0
Net income (loss)									52.5	53.7	50.2
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									84.3	80.7	76.9
Interest Expense									37.1	38.8	38.8
Income Taxes									34.5	35.1	31.3
Capital Expenditures									113.5	88.7	121.1
Assets	2,125.2				2,161.7				2,125.2	2,161.7	2,102.4
Utility Group [Member] | Electric Utility Services [Member]
Segment Reporting Information [Line Items]
Revenues									635.9	608.0	528.6
Net income (loss)									65.0	60.9	48.3
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									80.2	80.8	77.5
Interest Expense									36.4	36.4	34.8
Income Taxes									45.3	40.8	27.4
Capital Expenditures									102.2	120.1	154.1
Assets	1,656.5				1,666.5				1,656.5	1,666.5	1,592.4
Utility Group [Member] | Other Operations [Member]
Segment Reporting Information [Line Items]
Revenues									43.9	44.5	42.8
Net income (loss)									5.4	9.3	8.9
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									27.8	26.7	26.5
Interest Expense									6.8	6.2	5.6
Income Taxes									3.1	1.2	0.5
Capital Expenditures									17.8	22.5	16.7
Assets	192.8				96.3				192.8	96.3	128.3
Utility Group [Member] | Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues									(41.9)	(42.9)	(41.2)
Utility Group [Member] | Non-Cash Cost and Change in Accruals [Member]
Amounts Included in Profitability Measures [Abstract]
Capital Expenditures									(0.1)	(6.2)	10.8
Nonutility Group [Member]
Segment Reporting Information [Line Items]
Revenues									1,018.6	735.2	673.9
Net income (loss)									23.8	9.8	25.8
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									52.0	40.9	31.0
Interest Expense									27.0	23.6	20.9
Income Taxes									6.3	(1.5)	5.9
Capital Expenditures									87.9	52.1	129.3
Assets	888.3				868.5				888.3	868.5	836.0
Nonutility Group [Member] | Intersegment Elimination [Member]
Amounts Included in Profitability Measures [Abstract]
Assets	7.2				(2.2)				7.2	(2.2)	2.0
Nonutility Group [Member] | Infrastructure Services [Member]
Segment Reporting Information [Line Items]
Revenues									421.3	235.6	202.0
Net income (loss)									14.9	3.1	2.4
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									14.9	8.8	8.3
Interest Expense									7.4	3.3	2.6
Income Taxes									10.7	2.7	2.1
Capital Expenditures									22.8	12.0	11.0
Assets	295.0				174.6				295.0	174.6	141.4
Nonutility Group [Member] | Energy Services [Member]
Segment Reporting Information [Line Items]
Revenues									161.8	146.9	121.3
Net income (loss)									6.7	6.4	8.4
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									1.5	1.2	1.2
Interest Expense									0.6	0.2	0.6
Income Taxes									1.1	2.5	1.6
Capital Expenditures									9.7	1.2	1.9
Assets	81.2				67.4				81.2	67.4	54.5
Nonutility Group [Member] | Coal Mining [Member]
Segment Reporting Information [Line Items]
Revenues									285.6	209.9	193.4
Net income (loss)									16.6	11.9	13.4
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									35.1	30.4	21.0
Interest Expense									11.3	10.1	8.1
Income Taxes									3.9	1.9	4.1
Capital Expenditures									55.1	38.7	126.8
Assets	352.8				362.5				352.8	362.5	342.8
Nonutility Group [Member] | Energy Marketing [Member]
Segment Reporting Information [Line Items]
Revenues									149.9	142.8	157.2
Net income (loss)									(4.2)	(4.2)	4.1
Amounts Included in Profitability Measures [Abstract]
Depreciation and Amortization									0.5	0.5	0.5
Interest Expense									6.4	8.5	8.3
Income Taxes									(2.4)	(2.7)	0.3
Capital Expenditures									0.3	0.2	0.6
Assets	112.5				209.1				112.5	209.1	229.6
Nonutility Group [Member] | Other Businesses [Member]
Segment Reporting Information [Line Items]
Net income (loss)									(10.2)	(7.4)	(2.5)
Amounts Included in Profitability Measures [Abstract]
Interest Expense									1.3	1.5	1.3
Income Taxes									(7.0)	(5.9)	(2.2)
Capital Expenditures									0	0	(11.0)
Assets	39.6				57.1				39.6	57.1	65.7
Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(150.4)	(169.4)	(181.2)
Amounts Included in Profitability Measures [Abstract]
Assets	(711.2)				(735.0)				(711.2)	(735.0)	(703.2)
Corporate and Other Group [Member]
Segment Reporting Information [Line Items]
Net income (loss)									(5.1)	0	(0.1)
Amounts Included in Profitability Measures [Abstract]
Interest Expense									(0.8)	(0.4)	(0.1)
Income Taxes									(2.8)	(0.9)	(1.0)
Assets	$ 727.3				$ 706.2				$ 727.3	$ 706.2	$ 715.9