INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income statement data
Operating revenues	$ 165.5	$ 176.6	$ 330.9	$ 351.3
Operating expenses	91.5	83.0	176.4	165.7
Other expense, net	25.4	25.4	53.0	51.5
Net income	48.6	$ 68.2	101.5	$ 134.1
Balance sheet data
Current assets	97.1		97.1		$ 87.7
Noncurrent assets	4,764.3		4,764.3		4,598.9
Total assets	4,861.4		4,861.4		4,686.6
Current liabilities	700.3		700.3		767.2
Long-term debt	1,914.3		1,914.3		1,790.6
Other noncurrent liabilities	288.0		288.0		240.3
Shareholders' equity	1,958.8		1,958.8		1,888.5
Total liabilities and shareholders' equity	$ 4,861.4		$ 4,861.4		$ 4,686.6