Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net income		$ 128.7	$ 138.8
Gas		331.5	469.1
Unregulated Operating Revenue		13.4	12.4
Regulated and Unregulated Operating Revenue		1,056.7	1,223.0
Affiliate Revenue		1.1	1.1
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		711.9	741.6
Net income		74.0	82.4
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		331.4	469.2
Net income		52.1	59.2
All Other
Segment Reporting Information [Line Items]
Net income		2.6	(2.8)
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]	1,056.7	1,223.2
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric		711.8	741.5
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Gas		331.5	469.1
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	[1]	13.4	12.4
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue		0.0	(0.2)
Intersegment Eliminations | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric		0.1	0.1
Intersegment Eliminations | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Gas		$ (0.1)	$ 0.1

[1] Operating revenues include $1.1 million of other affiliate revenue for the three months ended March 31, 2020 and 2019.