Summary of Interest Expense and Interest Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Interest Expense
Long-term Debt	$ 20.2		$ 20.3		$ 20.3
Short-term Debt	1.2		1.5		1.7
Regulatory Liabilities	0.6		0.5
Interest Expense	22.0		22.3		22.0
Interest Income
Interest income	(3.2)		(4.2)		(1.6)
Total Interest Expense, net	18.8		18.1		20.4
Regulatory Asset
Interest Income
Interest income	(2.3)		(3.4)		(1.1)
AFUDC and Other
Interest Income
Interest income	$ (0.9)	[1]	$ (0.8)	[1]	$ (0.5)	[1]

[1] AFUDC-Allowance for Funds Used During Construction