INVESTMENTS AND FAIR VALUE MEASUREMENTS - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Total	$ 53,902	$ 12,367
Quoted Prices in Active Market (Level 1)
Assets:
Total	35,096	8,111
Significant Other Observable Inputs (Level 2)
Assets:
Total	18,806	4,256
Significant Unobservable Inputs (Level 3)
Assets:
Total	0	0
Fair Value, Measurements, Recurring \| Money market funds
Assets:
Cash equivalents	11,588	8,111
Fair Value, Measurements, Recurring \| Money market funds \| Quoted Prices in Active Market (Level 1)
Assets:
Cash equivalents	11,588	8,111
Fair Value, Measurements, Recurring \| Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Fair Value, Measurements, Recurring \| Money market funds \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Money market funds \| Fair Value, Measurements, Recurring
Assets:
Funds held for clients	23,508	0
Money market funds \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Market (Level 1)
Assets:
Funds held for clients	23,508	0
Money market funds \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Funds held for clients	0	0
Money market funds \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Funds held for clients	0	0
Available-for-sale securities \| Fair Value, Measurements, Recurring
Assets:
Funds held for clients	18,806	4,256
Available-for-sale securities \| Fair Value, Measurements, Recurring \| Quoted Prices in Active Market (Level 1)
Assets:
Funds held for clients	0	0
Available-for-sale securities \| Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Assets:
Funds held for clients	18,806	4,256
Available-for-sale securities \| Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Assets:
Funds held for clients	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: