FAIR VALUE MEASUREMENTS - Changes in the fair value (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Changes in the fair value of estimated contingent consideration for business acquisitions
Balance at beginning of period	$ 98,905	$ 76,677
Additions to estimated contingent consideration	29,583	42,086
Payments of contingent consideration	(16,819)	(26,237)
Non-cash changes in fair value of estimated contingent consideration	7,414	6,638
Other	(41)	(259)
Balance at end of period	119,042	98,905
Other liabilities
Changes in the fair value of estimated contingent consideration for business acquisitions
Contingent consideration paid in cash	$ 16,819	23,816
Contingent consideration issued of restricted common units		$ 2,421

Represents Fair Value additions to estimated contingent consideration.

No definition available.

Name:	focs_AdditionsToEstimatedContingentConsideration
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the fair value of estimated contingent consideration for business acquisitions.

No definition available.

Name:	focs_FairValueEstimatedContingentConsiderationBusinessAcquisition
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the fair value of estimated contingent consideration that is recorded in other.

No definition available.

Name:	focs_FairValueOfEstimatedContingentConsiderationOther
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the issued of contingent consideration of restricted common units.

No definition available.

Name:	focs_IssuedContingentConsiderationOfRestrictedCommonUnits
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the non-cash changes in fair value of estimated contingent consideration.

No definition available.

Name:	focs_NonCashChangesInFairValueOfEstimatedContingentConsideration
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents fair value payments of contingent consideration.

No definition available.

Name:	focs_PaymentsOfContingentConsideration
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents fair value payments of contingent consideration in cash.

No definition available.

Name:	focs_PaymentsOfContingentConsiderationInCash
Namespace Prefix:	focs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
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Balance Type:
Period Type: