SUMMARY OF SIGNIFCANT ACCOUNTING POLICIES - Redeemable noncontrolling interest (Details)	12 Months Ended
Dec. 31, 2018 $ / shares
Class C Common Stock
Class of Stock [Line Items]
Common stock, par value (in USD per share)	$ 0.0001
Class A Common Stock
Class of Stock [Line Items]
Common stock, par value (in USD per share)	$ 0.0001
Common stock, redemption ratio	1

Face amount or stated value per share of common stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant