SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible assets (Details)	12 Months Ended
Dec. 31, 2021
Customer Contracts \| Minimum
Intangible assets
Weighted-average amortization period (in years)	5 years
Customer Contracts \| Maximum
Intangible assets
Weighted-average amortization period (in years)	15 years
Licenses
Intangible assets
Weighted-average amortization period (in years)	20 years

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration