REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Regulatory liabilities
Other current liabilities	$ 126.1	$ 87.6
Regulatory liabilities	3,968.5	3,992.8
Total regulatory liabilities	4,094.6	4,080.4
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,192.5	2,248.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,202.4	1,181.5
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	343.5	354.9
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	138.9	89.8
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	66.4	42.2
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	38.2	43.5
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	33.2	39.1
Decoupling
Regulatory liabilities
Total regulatory liabilities	28.0	36.8
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	27.3	30.7
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 24.2	$ 13.1