Related Parties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Related Party Transaction [Line Items]
Income (Loss) from Equity Method Investments	$ 68.4	$ 65.5	$ 61.6
American Transmission Company [Member]
Provided and received services from the associated companies
Services Received, -ATC	234.2	222.7	219.2
Receivable and payable balances with ATC
Services Received, -ATC	19.5	18.6
Related Parties (Textuals) [Abstract]
Equity interest in ATC	26.20%
Services [Member] | American Transmission Company [Member]
Provided and received services from the associated companies
Services Provided, -ATC	9.0	8.2	10.8
Receivable and payable balances with ATC
Services Provided, -ATC	0.6	0.5
Transmission Affiliate [Member]
Related Party Transaction [Line Items]
Income (Loss) from Equity Method Investments	68.5	65.7	62.5
Distributions from transmission affiliate	$ 54.5	$ 52.6	$ 49.7