Investment in American Transmission Company - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment in ATC
Charges to ATC for services and construction	$ 15.4	$ 8.1	$ 9.0
Charges from ATC for network transmission services	289.2	231.4	$ 234.2
Accounts receivable for services provided to ATC	1.0	0.6
Accounts payable for services received from ATC	$ 28.3	$ 19.3