Investment in American Transmission Company - Related Party Transactions (Details) - ATC - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Investment in ATC
Charges to ATC for services and construction	$ 4.3	$ 2.5	$ 8.4	$ 5.0
Charges from ATC for network transmission services	91.1	$ 59.6	182.1	$ 119.2
Accounts receivable for services provided to ATC	1.4		1.4		$ 1.0
Accounts payable for services received from ATC	$ 30.3		$ 30.3		$ 28.3