Claims, Lawsuits, and Other Contingencies - Legal Narrative (Details) £ in Millions, $ in Millions						1 Months Ended		3 Months Ended								12 Months Ended
	Sep. 06, 2018 USD ($)	Jan. 26, 2018 USD ($)	Oct. 03, 2017 NZD ($)	Jun. 29, 2015 NZD ($)	Sep. 14, 2010 USD ($)	Aug. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 20, 2016 GBP (£)	Apr. 30, 2014 GBP (£)
Loss Contingencies [Line Items]
Total revenue								$ 2,770,000,000	$ 2,349,000,000	$ 2,561,000,000	$ 3,090,000,000	$ 2,909,000,000	$ 2,340,000,000	$ 2,368,000,000	$ 2,381,000,000	$ 10,770,000,000	$ 9,998,000,000	$ 9,409,000,000
Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss								57,000,000								57,000,000				£ 45
Trustees of Gleeds Pension Fund 2016 | Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss								88,000,000								88,000,000			£ 70
Opry Mills Mall Limited Partnership
Loss Contingencies [Line Items]
Value of damages sought					$ 200,000,000
Damages awarded		$ 50,000,000				$ 204,000,000	$ 200,000,000
Amount of coverage for damages contended by the insurers					50,000,000
Difference amount of damages sought by the client					$ 150,000,000
Pending Litigation | Lyttleton Port Company Limited
Loss Contingencies [Line Items]
Value of damages sought				$ 184												124,000,000
Pending Litigation | Christchurch City Council
Loss Contingencies [Line Items]
Value of damages sought			$ 528													355,000,000
Minimum
Loss Contingencies [Line Items]
Estimate of possible loss								0								0
Minimum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought	$ 45,000,000
Maximum
Loss Contingencies [Line Items]
Estimate of possible loss								$ 200,000,000								$ 200,000,000
Maximum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought	85,000,000
Damages awarded	$ 15,000,000
Aviation and Aerospace Broking Industry
Loss Contingencies [Line Items]
Total revenue																		$ 100,000,000