Summary of Interest Expense and Interest Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Interest Expense
Long-Term Debt	$ 20.5		$ 20.2		$ 20.3
Short-Term Debt	1.1		1.2		1.5
Regulatory Liabilities	0.6		0.6		0.5
Interest Expense	22.2		22.0		22.3
Interest Income
Interest income	(1.3)		(3.2)		(4.2)
Total Interest Expense, net	20.9		18.8		18.1
Regulatory Asset
Interest Income
Interest income	(0.6)		(2.3)		(3.4)
AFUDC and Other
Interest Income
Interest income	$ (0.7)	[1]	$ (0.9)	[1]	$ (0.8)	[1]

[1] AFUDC-Allowance for Funds Used During Construction