Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Customer revenues		$ 855.8	[1]	$ 883.4	$ 3,555.1	[1]	$ 3,386.0
Other revenues		39.2		33.6	97.7		120.3
Total Operating Revenues		895.0		$ 917.0	3,652.8		$ 3,506.3
Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	406.5			2,317.6
Other revenues		12.1			30.2
Total Operating Revenues		418.6			2,347.8
Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	449.1			1,236.9
Other revenues		27.1			67.5
Total Operating Revenues		476.2			1,304.4
Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.2			0.6
Other revenues		0.0			0.0
Total Operating Revenues		0.2			0.6
Residential
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	411.7
Residential | Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	257.0			1,540.3
Residential | Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	154.7			382.3
Residential | Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.0			0.0
Commercial | Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	80.9			516.2
Commercial | Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	140.7			374.2
Commercial | Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.0			0.0
Industrial | Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	39.0			161.3
Industrial | Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	153.6			468.1
Industrial | Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.0			0.0
Off-system | Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	20.4			63.6
Off-system | Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.0			0.0
Off-system | Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.0			0.0
Miscellaneous | Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	9.2			36.2
Miscellaneous | Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.1			12.3
Miscellaneous | Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	0.2			0.6
Residential
Disaggregation of Revenue [Line Items]
Customer revenues	[1]				1,922.6
Commercial
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	221.6			890.4
Industrial
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	192.6			629.4
Off-system
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	20.4			63.6
Miscellaneous
Disaggregation of Revenue [Line Items]
Customer revenues	[1]	$ 9.5			$ 49.1

[1] Customer revenue amounts exclude intersegment revenues. See Note 19, "Business Segment Information," for discussion of intersegment revenues.