Segment Reporting - Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 711.0	$ 691.2	$ 630.7	$ 624.5	$ 699.0	$ 631.0	$ 533.7	$ 584.8	$ 2,657.3	$ 2,448.3	$ 2,434.7
Profitability Measures - Net Income	61.2	$ 61.9	$ 37.6	$ 55.4	69.6	$ 61.4	$ 32.3	$ 48.3	216.0	211.6	197.3
Amounts Included in Profitability Measures
Depreciation & Amortization									276.2	260.0	256.3
Interest Expense									87.7	85.5	84.5
Income Taxes									46.4	112.9	99.7
Capital Expenditures									602.6	542.0	476.9
Assets	6,239.3				5,800.7				6,239.3	5,800.7	5,400.0
Utility Group
Segment Reporting Information [Line Items]
Revenues									1,382.6	1,377.8	1,394.5
Operating Segments | Utility Group
Segment Reporting Information [Line Items]
Profitability Measures - Net Income									175.8	173.6	160.9
Amounts Included in Profitability Measures
Depreciation & Amortization									234.5	219.1	208.8
Interest Expense									72.6	69.7	66.3
Income Taxes									60.7	99.5	88.1
Capital Expenditures									550.9	496.8	398.3
Assets	5,498.2				5,040.9				5,498.2	5,040.9	4,592.7
Operating Segments | Utility Group | Gas Utility Services
Segment Reporting Information [Line Items]
Revenues									812.7	771.7	792.6
Profitability Measures - Net Income									115.5	76.1	64.4
Amounts Included in Profitability Measures
Depreciation & Amortization									118.9	108.1	98.6
Interest Expense									43.0	40.1	35.8
Income Taxes									25.4	47.1	40.8
Capital Expenditures									391.4	358.5	291.2
Assets	3,457.8				3,091.0				3,457.8	3,091.0	2,706.9
Operating Segments | Utility Group | Electric Utility Services
Segment Reporting Information [Line Items]
Revenues									569.6	605.8	601.6
Profitability Measures - Net Income									75.2	84.7	82.6
Amounts Included in Profitability Measures
Depreciation & Amortization									89.5	87.1	85.6
Interest Expense									25.8	27.0	27.8
Income Taxes									41.4	50.1	49.3
Capital Expenditures									105.3	106.4	87.6
Assets	1,820.3				1,788.4				1,820.3	1,788.4	1,778.3
Operating Segments | Utility Group | Other Operations
Segment Reporting Information [Line Items]
Revenues									45.6	42.2	40.7
Profitability Measures - Net Income									(14.9)	12.8	13.9
Amounts Included in Profitability Measures
Depreciation & Amortization									26.1	23.9	24.6
Interest Expense									3.8	2.6	2.7
Income Taxes									(6.1)	2.3	(2.0)
Capital Expenditures									57.9	39.0	25.7
Assets	220.1				161.5				220.1	161.5	107.5
Operating Segments | Utility Group | Non-cash costs & changes in accruals
Amounts Included in Profitability Measures
Capital Expenditures									(3.7)	(7.1)	(6.2)
Operating Segments | Nonutility Group
Segment Reporting Information [Line Items]
Revenues									1,277.9	1,073.3	1,043.2
Profitability Measures - Net Income									41.1	36.9	36.3
Amounts Included in Profitability Measures
Depreciation & Amortization									41.7	40.9	47.5
Interest Expense									15.4	15.6	18.4
Income Taxes									(13.5)	14.7	13.4
Capital Expenditures									51.7	45.2	78.6
Assets	767.5				749.9				767.5	749.9	778.8
Operating Segments | Nonutility Group | Infrastructure Services
Segment Reporting Information [Line Items]
Revenues									996.1	813.3	843.3
Profitability Measures - Net Income									32.3	25.0	29.7
Amounts Included in Profitability Measures
Depreciation & Amortization									39.7	38.2	44.5
Interest Expense									13.8	12.8	16.0
Income Taxes									(12.9)	17.9	19.6
Capital Expenditures									48.4	43.2	78.1
Assets	552.6				513.9				552.6	513.9	554.5
Operating Segments | Nonutility Group | Energy Services
Segment Reporting Information [Line Items]
Revenues									281.8	260.0	199.9
Profitability Measures - Net Income									10.7	12.5	7.3
Amounts Included in Profitability Measures
Depreciation & Amortization									1.9	2.5	2.7
Interest Expense									0.6	1.9	1.2
Income Taxes									(1.5)	(3.5)	(7.7)
Capital Expenditures									3.2	1.8	0.5
Assets	155.8				182.7				155.8	182.7	160.3
Operating Segments | Nonutility Group | Other Businesses
Segment Reporting Information [Line Items]
Profitability Measures - Net Income									(1.9)	(0.6)	(0.7)
Amounts Included in Profitability Measures
Depreciation & Amortization									0.1	0.2	0.3
Interest Expense									1.0	0.9	1.2
Income Taxes									0.9	0.3	1.5
Capital Expenditures									0.1	0.2	0.0
Assets	59.1				53.3				59.1	53.3	64.0
Eliminations | Utility Group
Segment Reporting Information [Line Items]
Revenues									(45.3)	(41.9)	(40.4)
Eliminations
Segment Reporting Information [Line Items]
Revenues									(3.2)	(2.8)	(3.0)
Amounts Included in Profitability Measures
Assets	(475.5)				(618.5)				(475.5)	(618.5)	(713.9)
Corporate & Other
Segment Reporting Information [Line Items]
Profitability Measures - Net Income									(0.9)	1.1	0.1
Amounts Included in Profitability Measures
Interest Expense									(0.3)	0.2	(0.2)
Income Taxes									(0.8)	(1.3)	(1.8)
Assets	$ 449.1				$ 628.4				$ 449.1	$ 628.4	$ 742.4