Fiduciary Funds - Additional Information (Detail) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016
Uncategorized [Abstract]
Client deposit held	$ 4.0	$ 3.4

The amount of funds held by the company on behalf of clients as of the balance sheet date. Such funds are not included in the reporting entity's consolidated balance sheet as the reporting entity does not own or have any legal rights to use these funds.

No definition available.

Name:	cbg_FiduciaryFunds
Namespace Prefix:	cbg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant