INVESTMENT IN AMERICAN TRANSMISSION COMPANY - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income statement data
Revenues	$ 171.1	$ 158.1	$ 522.4	$ 476.6
Operating expenses	85.0	80.2	250.1	241.0
Other expense	27.5	23.5	79.6	71.2
Net income	58.6	$ 54.4	192.7	$ 164.4
Balance sheet data
Current assets	89.0		89.0		$ 75.8
Noncurrent assets	4,564.9		4,564.9		4,312.9
Total assets	4,653.9		4,653.9		4,388.7
Current liabilities	772.1		772.1		495.1
Long-term debt	1,740.8		1,740.8		1,865.3
Other noncurrent liabilities	213.8		213.8		271.5
Shareholders' equity	1,927.2		1,927.2		1,756.8
Total liabilities and shareholders' equity	$ 4,653.9		$ 4,653.9		$ 4,388.7