SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Grants (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Minimum
Accounting for grants
Long-term power purchase agreements term	10 years
Maximum
Accounting for grants
Long-term power purchase agreements term	25 years
Mobility Funds
Accounting for grants
Mobility Funds Received	$ 21.1