Regulatory Assets (Detail) - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Regulatory Assets [Line Items]
Regulatory Assets		$ 117.2	$ 109.6	$ 118.0
Less: Current Portion of Regulatory Assets	[1]	65.1	56.5	66.4
Regulatory Assets – noncurrent		52.1	53.1	51.6
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		6.2	6.1	6.2
Other Deferred Charges
Regulatory Assets [Line Items]
Regulatory Assets		12.6	11.0	11.2
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		27.4	29.8	28.0
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		7.8	9.2	7.7
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		0.9	1.1	1.6
Energy Supply and Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 62.3	$ 52.4	$ 63.3

[1] Reflects amounts included in the Accrued Revenue on the Company’s Consolidated Balance Sheets.