INVESTMENT IN AMERICAN TRANSMISSION COMPANY - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Income statement data
Revenues	$ 174.7	$ 164.2
Operating expenses	82.4	79.1
Other expense	26.4	24.0
Net income	65.9	$ 61.1
Balance sheet data
Current assets	85.5		$ 75.8
Noncurrent assets	4,402.2		4,312.9
Total assets	4,487.7		4,388.7
Current liabilities	401.8		495.1
Long-term debt	1,940.4		1,865.3
Other noncurrent liabilities	282.9		271.5
Shareholders' equity	1,862.6		1,756.8
Total liabilities and shareholders' equity	$ 4,487.7		$ 4,388.7