Regulatory Assets (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Regulatory Assets [Line Items]
Regulatory Assets		$ 145.8	$ 149.1	$ 135.5
Less: Current Portion of Regulatory Assets	[1]	34.6	39.5	32.0
Regulatory Assets - noncurrent		111.2	109.6	103.5
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		9.0	9.5	10.7
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets		5.2	5.4	5.5
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		85.4	84.5	75.7
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		8.0	7.2	8.6
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		6.3	6.5	7.3
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 31.9	$ 36.0	$ 27.7

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