Commitments and Contingencies - Legal Narrative (Details) £ in Millions, SFr in Millions, NZD in Millions									1 Months Ended				12 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 29, 2015 NZD	Jan. 26, 2015 USD ($)	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 ($)	Jun. 30, 2015 GBP (£)	Mar. 31, 2015 USD ($)	May 31, 2010 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 20, 2016 GBP (£)	Apr. 30, 2014 GBP (£)
Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss	$ 55,000,000													$ 55,000,000		£ 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 46.0						45,000,000
Damages awarded excluding interest and costs						SFr 16.8								16,000,000
Damages awarded						SFr 27.9								27,000,000
Litigation settlement			$ 14,000,000	SFr 12.8
Settlement agreement terms maximum liability	8,000,000			SFr 8.7
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					$ 3,000,000
Damages awarded					$ 5,000,000
Litigation settlement			4,700,000
Settlement agreement terms maximum liability			$ 115,000
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	86,000,000													86,000,000	£ 70
Philips
Loss Contingencies [Line Items]
Value of damages sought													£ 189	232,000,000
Philips 2015
Loss Contingencies [Line Items]
Value of damages sought										£ 290				356,000,000
Pending Litigation | Lyttleton Port Company Limited
Loss Contingencies [Line Items]
Value of damages sought		NZD 184												127,000,000
Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	0													0
Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	$ 200,000,000													$ 200,000,000