REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 103.9	$ 36.8
Regulatory liabilities	4,274.3	4,251.6
Total regulatory liabilities	4,378.2	4,288.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,401.2	2,406.6
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,330.8	1,329.6
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	235.7	238.3
Mines deferral
Regulatory liabilities
Total regulatory liabilities	129.1	120.8
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	103.0	39.6
Decoupling
Regulatory liabilities
Total regulatory liabilities	51.0	30.5
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	45.7	31.7
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	29.9	30.0
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	29.5	30.5
Derivatives
Regulatory liabilities
Total regulatory liabilities	9.0	16.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 13.3	$ 14.4