Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Customer revenues	$ 952.4	$ 932.7	$ 2,458.9	$ 2,458.6
Other revenues	33.6	30.0	72.7	109.6
Total Operating Revenues	986.0	962.7	2,531.6	2,568.2
Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	569.9	599.3	1,697.0	1,770.6
Other revenues	4.4	7.0	13.1	63.7
Total Operating Revenues	574.3	606.3	1,710.1	1,834.3
Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	382.3	333.2	761.5	687.6
Other revenues	21.2	23.0	44.5	45.9
Total Operating Revenues	403.5	356.2	806.0	733.5
Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.2	0.2	0.4	0.4
Other revenues	8.0	0.0	15.1	0.0
Total Operating Revenues	8.2	0.2	15.5	0.4
Residential
Disaggregation of Revenue [Line Items]
Customer revenues	510.5	542.2	1,413.2	1,457.9
Residential \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	378.6	414.7	1,152.1	1,211.2
Residential \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	131.9	127.5	261.1	246.7
Residential \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Commercial
Disaggregation of Revenue [Line Items]
Customer revenues	255.4	235.6	649.7	625.2
Commercial \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	125.4	122.7	396.9	392.1
Commercial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	130.0	112.9	252.8	233.1
Commercial \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Industrial
Disaggregation of Revenue [Line Items]
Customer revenues	163.0	137.9	343.8	321.2
Industrial \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	44.0	48.6	101.9	122.8
Industrial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	119.0	89.3	241.9	198.4
Industrial \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Off-system
Disaggregation of Revenue [Line Items]
Customer revenues	17.0	8.0	31.4	26.7
Off-system \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	17.0	8.0	31.4	26.7
Off-system \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Off-system \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Miscellaneous
Disaggregation of Revenue [Line Items]
Customer revenues	6.5	9.0	20.8	27.6
Miscellaneous \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	4.9	5.3	14.7	17.8
Miscellaneous \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	1.4	3.5	5.7	9.4
Miscellaneous \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	$ 0.2	$ 0.2	$ 0.4	$ 0.4