SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Revenue recognition (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue recognition
Inventories relating to unrecognized revenue	$ 9.0	$ 7.1	$ 5.9
Performance-based energy incentives recognized	4.7	10.9	22.8
Total lease income recognized related to PPAs	$ 1.5	$ 2.5	$ 6.2

The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e39896-112707

Name:	us-gaap_OperatingLeasesIncomeStatementLeaseRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration