Segments and Related Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										$ 1,021.5	$ 1,005.5	$ 957.2
Depreciation and amortization										126.1	111.2	98.3
Total interest charges and financing costs										35.2	32.2	32.6
Income tax expense (benefit)										32.3	44.2	42.9
Net income (loss)		$ 20.4	$ 31.0	$ 15.2	$ 31.4	$ 20.4	$ 22.3	$ 14.3	$ 22.4	98.0	79.4	69.1
Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										879.0	882.4	850.3
Intercompany Revenue										157.9	177.2	170.5
Depreciation and amortization										97.8	88.9	81.3
Total interest charges and financing costs										31.8	29.4	29.7
Income tax expense (benefit)										28.4	38.9	40.5
Net income (loss)										85.5	70.9	65.0
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										142.0	122.7	106.7
Depreciation and amortization										28.1	22.1	16.8
Total interest charges and financing costs										3.3	2.8	2.9
Income tax expense (benefit)										4.5	4.0	2.4
Net income (loss)										12.5	7.8	4.5
All Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]									1.3	1.2	1.1
Depreciation and amortization										0.2	0.2	0.2
Total interest charges and financing costs										0.1	0.0	0.0
Income tax expense (benefit)										(0.6)	1.3	0.0
Net income (loss)										0.0	0.7	(0.4)
Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,022.3	1,006.3	958.1
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]									878.6	881.9	849.9
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]									141.6	122.4	106.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										(0.8)	(0.8)	(0.9)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										0.4	0.5	0.4
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										$ 0.4	$ 0.3	$ 0.5

[1] (a) Operating revenues include $157.9 million, $177.2 million and $170.5 million of intercompany revenue for the years ended Dec. 31, 2018, 2017 and 2016, respectively. See Note 13 for further information.