Regulatory Assets (Detail) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory Assets		$ 164.7	$ 147.8
Less: Current Portion of Regulatory Assets	[1]	37.3	35.8
Regulatory Assets—noncurrent		127.4	112.0
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		5.2	7.2
Other Deferred Charges
Regulatory Assets [Line Items]
Regulatory Assets		14.2	10.9
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		103.7	88.9
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		4.1	5.6
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		3.4	4.2
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 34.1	$ 31.0

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