REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Regulatory liabilities
Other current liabilities	$ 4.0	$ 11.9
Regulatory liabilities	2,005.5	2,002.3
Total regulatory liabilities	2,009.5	2,014.2
Income tax related items
Regulatory liabilities
Total regulatory liabilities	993.4	1,024.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	768.3	748.1
Mines deferral
Regulatory liabilities
Total regulatory liabilities	132.1	120.8
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	72.9	74.7
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	23.8	16.4
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	15.3	13.5
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 3.7	$ 15.9