Note 2 - Summary of Significant Accounting Policies (Details Textual)	12 Months Ended
Dec. 31, 2018
Long-term Incentive Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years
Minimum [Member] \| Long-term Incentive Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years
Minimum [Member] \| Customer Lists and Relationships [Member]
Finite-Lived Intangible Asset, Useful Life	4 years
Minimum [Member] \| Investment Management Contracts [Member]
Finite-Lived Intangible Asset, Useful Life	5 years
Minimum [Member] \| Trademarks and Trade Names [Member]
Finite-Lived Intangible Asset, Useful Life	2 years
Minimum [Member] \| Management Contracts and Other [Member]
Finite-Lived Intangible Asset, Useful Life	2 years
Maximum [Member] \| Long-term Incentive Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	10 years
Maximum [Member] \| Customer Lists and Relationships [Member]
Finite-Lived Intangible Asset, Useful Life	20 years
Maximum [Member] \| Investment Management Contracts [Member]
Finite-Lived Intangible Asset, Useful Life	15 years
Maximum [Member] \| Trademarks and Trade Names [Member]
Finite-Lived Intangible Asset, Useful Life	10 years
Maximum [Member] \| Management Contracts and Other [Member]
Finite-Lived Intangible Asset, Useful Life	10 years
Building [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life	20 years
Building [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life	40 years
Vehicles [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Vehicles [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life	5 years
Furniture and Fixtures [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Furniture and Fixtures [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years
Computer Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Computer Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life	5 years
Leasehold Improvements [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life	10 years

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

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

- Details

Name:	us-gaap_PlanNameAxis=cigi_LongTermIncentivePlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=cigi_CustomerListsAndRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=cigi_InvestmentManagementContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksAndTradeNamesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=cigi_ManagementContractsAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_VehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ComputerEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LeaseholdImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: