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LEASES - RIGHT OF USE ASSETS (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Jan. 01, 2019
Dec. 31, 2018
Right of use assets      
Operating lease right of use assets $ 48.2 $ 49.0  
Finance lease      
Right of use assets      
Long-term power purchase commitment 140.3   $ 140.3
Accumulated amortization (122.3)   (120.9)
Total finance lease right of use asset/capital lease asset $ 18.0   $ 19.4