LEASES - FUTURE MINIMUM LEASE PAYMENTS (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Total operating leases
Six months ended December 31, 2019	$ 2.7
2020	6.9
2021	4.9
2022	4.9
2023	5.0
2024	4.8
Thereafter	30.5
Total minimum lease payments	59.7
Less: interest	(15.3)
Present value of minimum lease payments	44.4	$ 48.8
Less: short-term lease liabilities	(4.3)
Long-term lease liabilities	40.1
Power purchase commitment
Six months ended December 31, 2019	4.1
2020	8.8
2021	9.4
2022	4.2
2023	0.0
2024	0.0
Thereafter	0.0
Total minimum lease payments	26.5
Less: interest	(5.5)
Present value of minimum lease payments	21.0
Less: short-term lease liabilities	(5.6)
Long-term lease liabilities	$ 15.4
Capital lease
Short-term liabilities under capital lease			$ 4.9
Long-term liabilities under capital lease			$ 18.4