Commitments and Contingencies - Legal Narrative (Details) £ in Millions, NZD in Millions												1 Months Ended					3 Months Ended								6 Months Ended	12 Months Ended
	Jan. 26, 2018 USD ($)	Oct. 03, 2017 USD ($)	Oct. 03, 2017 NZD	Jun. 29, 2015 NZD	Jan. 31, 2015 USD ($)	Jan. 26, 2015 USD ($)	Jan. 26, 2015 CHF (SFr)	Dec. 02, 2014 USD ($)	Dec. 02, 2014 CHF (SFr)	Jun. 01, 2007 USD ($)	Jun. 01, 2007 CHF (SFr)	Aug. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 CHF (SFr)	May 31, 2010 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 20, 2016 GBP (£)	Apr. 30, 2014 GBP (£)
Loss Contingencies [Line Items]
Total revenue																	$ 2,909,000,000	$ 2,340,000,000	$ 2,368,000,000	$ 2,381,000,000	$ 2,650,000,000	$ 2,201,000,000	$ 2,282,000,000	$ 2,276,000,000		$ 9,998,000,000	$ 9,409,000,000	$ 9,480,000,000
Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss																	60,000,000									60,000,000				£ 45
Opry Mills Mall Limited Partnership
Loss Contingencies [Line Items]
Value of damages sought																$ 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
Opry Mills Mall Limited Partnership
Loss Contingencies [Line Items]
Damages awarded												$ 204,000,000	$ 200,000,000
International Road Transport Union | Litigation Foreign Currency Denominated Award
Loss Contingencies [Line Items]
Value of damages sought											SFr 46,000,000														$ 47,000,000
Damages awarded excluding interest and costs									SFr 16,800,000																17,000,000
Damages awarded															SFr 27,900,000											28,000,000
Litigation settlement amount awarded to other party					$ 14,000,000		SFr 12,800,000
Settlement agreement terms maximum liability							SFr 15,000,000.0																		$ 15,000,000
International Road Transport Union | Litigation USD Denominated Award
Loss Contingencies [Line Items]
Value of damages sought										$ 3,000,000
Damages awarded excluding interest and costs								$ 3,100,000
Damages awarded														$ 5,000,000
Litigation settlement amount awarded to other party						$ 4,700,000
Settlement agreement terms maximum liability						$ 344,000
International Road Transport Union | Litigation Expenses and Interest
Loss Contingencies [Line Items]
Value of damages sought										$ 30,000,000
Trustees of Gleeds Pension Fund 2016 | Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss																	94,000,000									94,000,000			£ 70
Pending Litigation | Lyttleton Port Company Limited
Loss Contingencies [Line Items]
Value of damages sought				NZD 184																						130,000,000
Pending Litigation | Christchurch City Council
Loss Contingencies [Line Items]
Value of damages sought		$ 372,000,000	NZD 528
Minimum
Loss Contingencies [Line Items]
Estimate of possible loss																	0									0
Maximum
Loss Contingencies [Line Items]
Estimate of possible loss																	$ 300,000,000									$ 300,000,000
Aviation and Aerospace Broking Industry
Loss Contingencies [Line Items]
Total revenue																											$ 100,000,000
Subsequent Event | Opry Mills Mall Limited Partnership
Loss Contingencies [Line Items]
Damages awarded	$ 50,000,000