Investment in American Transmission Company - Summarized Financial Data (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Income statement data
Revenues	$ 158.1	$ 164.5	$ 476.6	$ 482.0
Operating expenses	80.2	78.0	241.0	238.3
Other expense	23.5	23.1	71.2	71.7
Net income	54.4	$ 63.4	164.4	$ 172.0
Balance sheet data
Current assets	86.3		86.3		$ 80.5
Noncurrent assets	4,205.7		4,205.7		3,948.3
Total assets	4,292.0		4,292.0		4,028.8
Current liabilities	420.7		420.7		330.3
Long-term debt	1,791.2		1,791.2		1,790.7
Other noncurrent liabilities	318.7		318.7		245.0
Shareholders' equity	1,761.4		1,761.4		1,662.8
Total liabilities and shareholders' equity	$ 4,292.0		$ 4,292.0		$ 4,028.8