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Note 15 - Warehouse Credit Facilities - Mortgage Warehouse Credit Facilities (Details) (Parentheticals) - Warehouse Credit Facilities [Member]	12 Months Ended
	Dec. 31, 2020
Facility A [Member] | London Interbank Offered Rate (LIBOR) [Member]
Basis spread	1.60%
Facility B [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Basis spread	1.70%

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_CreditFacilityAxis=cigi_WarehouseCreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cigi_FacilityAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cigi_FacilityBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: