Investment in American Transmission Company - Transactions with ATC (Details) - ATC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment in ATC
Charges to ATC for services and construction	$ 18.5	$ 15.4	$ 8.1
Charges from ATC for network transmission services	357.3	289.2	$ 231.4
Accounts receivable for services provided to ATC	2.2	1.0
Accounts payable for services received from ATC	$ 28.7	$ 28.3