Segments and Related Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenue, Regulated Natural Gas										$ 136.4	$ 141.6	$ 122.4
Unregulated Operating Revenue										0.4	1.3	1.2
Regulated and Unregulated Operating Revenue		$ 232.1	$ 243.7	$ 228.9	$ 276.2	$ 260.6	$ 256.0	$ 231.8	$ 273.1	980.9	1,021.5	1,005.5
Depreciation and amortization										137.5	126.1	111.2
Interest charges and financing costs										38.1	35.2	32.2
Income Tax Expense (Benefit)										15.7	32.3	44.2
Net income		$ 16.9	$ 29.4	$ 8.6	$ 24.0	$ 20.4	$ 31.0	$ 15.2	$ 31.4	78.9	98.0	79.4
Intercompany Revenue										176.6	157.9	177.2
Electricity, US Regulated
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										844.4	879.0	882.4
Depreciation and amortization										108.4	97.8	88.9
Interest charges and financing costs										34.2	31.8	29.4
Income Tax Expense (Benefit)										12.5	28.4	38.9
Net income										68.5	85.5	70.9
Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										136.8	142.0	122.7
Depreciation and amortization										29.0	28.1	22.1
Interest charges and financing costs										3.9	3.3	2.8
Income Tax Expense (Benefit)										2.2	4.5	4.0
Net income										9.9	12.5	7.8
All Other
Segment Reporting Information [Line Items]
Depreciation and amortization										0.1	0.2	0.2
Interest charges and financing costs										0.0	0.1	0.0
Income Tax Expense (Benefit)										1.0	(0.6)	1.3
Net income										0.5	0.0	0.7
Operating Segments
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]									981.6	1,022.3	1,006.3
Operating Segments | Electricity, US Regulated
Segment Reporting Information [Line Items]
Operating revenue, Regulated Electric	[1]									844.1	878.6	881.9
Operating Segments | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Operating revenue, Regulated Natural Gas	[1]									136.4	141.6	122.4
Operating Segments | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	[1]									0.4	1.3	1.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue										(0.7)	(0.8)	(0.8)
Intersegment Eliminations | Electricity, US Regulated
Segment Reporting Information [Line Items]
Operating revenue, Regulated Electric										0.3	0.4	0.5
Intersegment Eliminations | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Operating revenue, Regulated Natural Gas										$ 0.4	$ 0.4	$ 0.3

[1] ^(a)  Operating revenues include $176.6 million, $157.9 million and $177.2 million of intercompany revenue for the years ended Dec. 31, 2019, 2018 and 2017, respectively. See Note 13 for further information.