REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Regulatory liabilities
Other current liabilities	$ 49.4	$ 36.8
Regulatory liabilities	4,198.8	4,251.6
Total regulatory liabilities	4,248.2	4,288.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,358.6	2,406.6
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,313.3	1,329.6
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	227.1	238.3
Mines deferral
Regulatory liabilities
Total regulatory liabilities	132.1	120.8
Decoupling
Regulatory liabilities
Total regulatory liabilities	46.9	30.5
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	45.2	39.6
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	33.4	31.7
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	32.3	30.5
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	28.5	30.0
Derivatives
Regulatory liabilities
Total regulatory liabilities	18.6	16.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 12.2	$ 14.4