Investment in American Transmission Company - ATC Summarized Financial Data (Details) - ATC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income statement data
Revenues	$ 650.8	$ 615.8	$ 635.0
Operating expenses	322.5	319.3	307.4
Other expense	95.5	96.1	88.9
Net income	232.8	200.4	$ 238.7
Balance sheet data
Current assets	75.8	80.5
Noncurrent assets	4,312.9	3,948.3
Total assets	4,388.7	4,028.8
Current liabilities	495.1	330.3
Long-term debt	1,865.3	1,790.7
Other noncurrent liabilities	271.5	245.0
Shareholders' equity	1,756.8	1,662.8
Total liabilities and shareholders' equity	$ 4,388.7	$ 4,028.8