Regulatory Assets (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Regulatory Assets [Line Items]
Regulatory Assets		$ 138.0	$ 149.1	$ 133.3
Less: Current Portion of Regulatory Assets	[1]	26.4	39.5	31.3
Regulatory Assets - noncurrent		111.6	109.6	102.0
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		8.9	9.5	9.9
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets		4.1	5.4	5.5
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		88.0	84.5	76.2
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		6.3	7.2	6.8
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		5.9	6.5	6.7
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 24.8	$ 36.0	$ 28.2

[1] Reflects amounts included in Accrued Revenue, discussed above, on the Company's Consolidated Balance Sheets.