Subsequent Event - Additional Information (Detail) - 2017 Credit Agreement [Member] - USD ($)		9 Months Ended
Oct. 31, 2017	Sep. 30, 2017
Subsequent Event [Line Items]
Credit agreement date		Oct. 31, 2017
Tranche A term loan facility [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Amounts available to borrow under Credit Agreement	$ 750,000,000
Initial borrowing	200,000,000
Remaining amount in credit facility	$ 550,000,000
Tranche A term loan facility [Member] \| Subsequent Event [Member] \| Minimum [Member]
Subsequent Event [Line Items]
Credit Agreement applicable fixed rate spread	0.875%
Credit Agreement applicable daily rate spread	0.00%
Tranche A term loan facility [Member] \| Subsequent Event [Member] \| Maximum [Member]
Subsequent Event [Line Items]
Credit Agreement applicable fixed rate spread	1.25%
Credit Agreement applicable daily rate spread	0.25%
Leverage ratio	250.00%
Revolving credit facility [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Amounts available to borrow under Credit Agreement	$ 2,800,000,000
Remaining amount in credit facility	83,000,000
Revolving credit facility [Member] \| Subsequent Event [Member] \| Canadian, Australian and New Zealand subsidiaries [Member]
Subsequent Event [Line Items]
Amounts available to borrow under Credit Agreement	200,000,000
Revolving credit facility [Member] \| Subsequent Event [Member] \| U.K. subsidiaries [Member]
Subsequent Event [Line Items]
Amounts available to borrow under Credit Agreement	$ 300,000,000
Revolving credit facility [Member] \| Subsequent Event [Member] \| Minimum [Member]
Subsequent Event [Line Items]
Credit Agreement applicable fixed rate spread	0.775%
Credit Agreement applicable daily rate spread	0.00%
Revolving credit facility [Member] \| Subsequent Event [Member] \| Maximum [Member]
Subsequent Event [Line Items]
Credit Agreement applicable fixed rate spread	1.075%
Credit Agreement applicable daily rate spread	0.075%

Credit agreement date.

No definition available.

Name:	cbg_CreditAgreementDate
Namespace Prefix:	cbg_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Leverage ratio.

+ References

No definition available.

+ Details

Name:	cbg_LeverageRatio
Namespace Prefix:	cbg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long term debt percentage spread over daily interest rate

+ References

No definition available.

+ Details

Name:	cbg_LongTermDebtPercentageSpreadOverDailyInterestRate
Namespace Prefix:	cbg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long term debt percentage spread over fixed interest rate

+ References

No definition available.

+ Details

Name:	cbg_LongTermDebtPercentageSpreadOverFixedInterestRate
Namespace Prefix:	cbg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=cbg_CanadianAustralianAndNewZealandSubsidiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=cbg_UnitedKingdomSubsidiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: