Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
Regulatory Assets [Line Items]
Regulatory Assets	$ 109.9		$ 143.7		$ 153.9
Less: Current Portion of Regulatory Assets	24.6	[1]	43.6	[1]	33.0	[1]
Regulatory Assets - noncurrent	85.3		100.1		120.9
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets	10.6		16.1		16.1
Retirement Benefit
Regulatory Assets [Line Items]
Regulatory Assets	42.2		42.6		62.7
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets	10.1		11.9		9.3
Electric Utilities
Regulatory Assets [Line Items]
Regulatory Assets	20.7		25.6		26.5
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets	6.6		5.7		6.8
Energy Supply & Other Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	17.6		32.5		20.9
Deferred Restructuring Costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 2.1		$ 9.3		$ 11.6

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