Regulatory Assets (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Regulatory Assets [Line Items]
Regulatory Assets		$ 131.9	$ 126.4	$ 139.9
Less: Current Portion of Regulatory Assets	[1]	33.7	26.8	33.7
Regulatory Assets - noncurrent		98.2	99.6	106.2
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		11.9	11.2	11.0
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets		5.3	5.3	5.8
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		64.4	64.7	65.0
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		13.8	15.4	20.1
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		8.3	8.5	9.5
Energy Supply & Other Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 28.2	$ 21.3	$ 28.5

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