Acquisitions - Contingent earn-out obligation (Details) - Walker $ in Millions	Apr. 01, 2019 USD ($)
Business Acquisition [Line Items]
Contingent earn-out period	5 years
Contingent earn-out estimated milestone payment, minimum	$ 1
Contingent earn-out estimated milestone payment, maximum	$ 11
Cash flow discount rate	10.20%

Business Combination Contingent Consideration Arrangements Earn Out Period.

No definition available.

Name:	fix_BusinessCombinationContingentConsiderationArrangementsEarnOutPeriod
Namespace Prefix:	fix_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of probable cash flows discount rate.

+ References

No definition available.

+ Details

Name:	fix_CashFlowDiscountRate
Namespace Prefix:	fix_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For contingent consideration arrangements and indemnification assets recognized in connection with a business combination, this element represents an estimate of the low-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueLow
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fix_WalkerTxHoldingsInc.AndItsWhollyOwnedSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: