Regulatory Assets (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Regulatory Assets [Line Items]
Regulatory Assets		$ 126.9	$ 145.4	$ 115.4
Less: Current Portion of Regulatory Assets	[1]	22.6	37.8	27.3
Regulatory Assets - noncurrent		104.3	107.6	88.1
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		10.9	11.0	10.6
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		65.1	65.1	42.2
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		18.6	21.2	22.3
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		9.1	9.7	10.4
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets		5.8	5.8	7.5
Energy Supply & Other Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 17.4	31.0	17.7
Deferred Restructuring Costs
Regulatory Assets [Line Items]
Regulatory Assets			$ 1.6	$ 4.7

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