REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 86.8	$ 36.8
Regulatory liabilities	4,243.6	4,251.6
Total regulatory liabilities	4,330.4	4,288.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,389.6	2,406.6
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,304.7	1,329.6
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	231.2	238.3
Mines deferral
Regulatory liabilities
Total regulatory liabilities	130.5	120.8
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	90.1	39.6
Decoupling
Regulatory liabilities
Total regulatory liabilities	48.7	30.5
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	42.0	31.7
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	31.9	30.5
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	30.0	30.0
Derivatives
Regulatory liabilities
Total regulatory liabilities	17.5	16.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 14.2	$ 14.4