Regulatory Assets (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory Assets		$ 149.1	$ 142.0
Less: Current Portion of Regulatory Assets	[1]	39.5	37.9
Regulatory Assets-noncurrent		109.6	104.1
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		9.5	10.8
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets		5.4	5.7
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		84.5	75.9
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		7.2	9.6
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		6.5	7.3
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 36.0	$ 32.7

[1] Reflects amounts included in Accrued Revenue on the Company's Consolidated Balance Sheets and in the Accrued Revenue table shown above.