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	$ 10.0	$ 9.7	$ 8.1
Charges from ATC for network transmission services	247.8	238.5	$ 231.4
Accounts receivable for services provided to ATC	1.1	0.6
Accounts payable for services received from ATC	$ 20.0	$ 19.9