Components of Gas and Electric Operating Revenue (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	$ 178.7	$ 220.2
Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	171.4	219.5
Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	7.3	0.7
Electric
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	73.6	108.2
Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	71.0	97.6
Electric | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	2.6	10.6
Gas Segment
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	105.1	112.0
Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	100.4	121.9
Gas Segment | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	4.7	(9.9)
Residential
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	81.6	109.2
Residential | Electric
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	40.5	60.1
Residential | Gas Segment
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	41.1	49.1
Commercial & Industrial
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	83.3	104.0
Commercial & Industrial | Electric
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	28.1	34.8
Commercial & Industrial | Gas Segment
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	55.2	69.2
Other
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	6.5	6.3
Other | Electric
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	2.4	2.7
Other | Gas Segment
Operating Revenues [Line Items]
Total Electric and Gas Operating Revenues	$ 4.1	$ 3.6