INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income statement data
Operating revenues	$ 165.4	$ 174.7
Operating expenses	84.9	82.7
Other expense, net	27.6	26.1
Net income	52.9	$ 65.9
Balance sheet data
Current assets	84.7		$ 87.7
Noncurrent assets	4,681.7		4,598.9
Total assets	4,766.4		4,686.6
Current liabilities	473.3		767.2
Long-term debt	2,065.3		1,790.6
Other noncurrent liabilities	266.4		240.3
Shareholders' equity	1,961.4		1,888.5
Total liabilities and shareholders' equity	$ 4,766.4		$ 4,686.6