Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013
Regulatory Assets [Line Items]
Regulatory Assets	$ 136.2		$ 143.7		$ 167.0
Less: Current Portion of Regulatory Assets	39.7	[1]	43.6	[1]	37.0	[1]
Regulatory Assets - noncurrent	96.5		100.1		130.0
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets	16.0		16.1		16.7
Retirement Benefit
Regulatory Assets [Line Items]
Regulatory Assets	42.3		42.6		62.4
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets	10.7		11.9		10.0
Electric Utilities
Regulatory Assets [Line Items]
Regulatory Assets	24.2		25.6		27.7
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets	5.8		5.7		7.0
Energy Supply & Other Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	30.3		32.5		26.2
Deferred Restructuring Costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 6.9		$ 9.3		$ 17.0

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