Regulatory Assets (Detail) - USD ($) $ in Millions		Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Regulatory Assets		$ 156.3	$ 164.7
Less: Current Portion of Regulatory Assets	[1]	47.4	37.3
Regulatory Assets—noncurrent		108.9	127.4
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		4.6	5.2
Other Deferred Charges
Regulatory Assets [Line Items]
Regulatory Assets		15.3	14.2
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		86.4	103.7
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		3.3	4.1
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		2.6	3.4
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 44.1	$ 34.1

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