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Note 15 - Warehouse Credit Facilities - Mortgage Warehouse Credit Facilities (Details) (Parentheticals) - Warehouse Credit Facilities [Member] | 12 Months Ended
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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%

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- 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: |