Commitments and Contingencies - Narrative (Details)																												3 Months Ended		86 Months Ended
	Jun. 17, 2020 lawsuit	Jun. 12, 2020 USD ($)	May 28, 2020 lawsuit	May 19, 2020 lawsuit	May 14, 2020 lawsuit	May 11, 2020 lawsuit	Oct. 18, 2018 lawsuit	Mar. 09, 2018 lawsuit	Mar. 08, 2018 lawsuit	Feb. 27, 2018 lawsuit	Nov. 21, 2017 lawsuit	Sep. 12, 2016 USD ($) plaintiff	Aug. 05, 2016 USD ($) plaintiff	Mar. 31, 2016 USD ($)	Jul. 21, 2015	Jul. 15, 2015	Oct. 01, 2013 USD ($)	Jun. 20, 2013 lawsuit	Jun. 11, 2013 lawsuit	Jun. 03, 2013 lawsuit	Feb. 14, 2013 USD ($) lawsuit plaintiff	Feb. 08, 2013 USD ($) plaintiff	Apr. 01, 2011 USD ($)	Mar. 11, 2011 USD ($) plaintiff	Sep. 16, 2010 USD ($) plaintiff	Sep. 14, 2009 USD ($) plaintiff	Aug. 06, 2009 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 12, 2016 lawsuit	Mar. 25, 2014 action
Legacy Towers Watson Stockholders [Member] | Pending Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit											1
Legacy Towers Watson, City of Fort Myers General Employees Pension Fund (Fort Myers) [Member] | Pending Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit										1
Alaska Laborers-Employers Retirement Trust (Alaska) [Member] | Pending Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit									1
Regents [Member] | Pending Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit								1
Legacy Towers Watson, Naya Master Fund LP, Naya 174 Fund Limited and Naya Lincoln Park Master Fund Limited (collectively, Naya) [Member] [Member] | Dismissed Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit							3
Stanford Financial Group [Member] | Pending Litigation [Member] | Canabal, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																											$ 1,000,000,000
Stanford Financial Group [Member] | Pending Litigation [Member] | Rupert, et al. v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																										$ 300,000,000
Number of plaintiffs | plaintiff																										97
Stanford Financial Group [Member] | Pending Litigation [Member] | Casanova, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																									$ 5,000,000
Number of plaintiffs | plaintiff																									7
Stanford Financial Group [Member] | Pending Litigation [Member] | Rishmague, et ano. v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																								$ 37,000,000
Number of plaintiffs | plaintiff																								2
Stanford Financial Group [Member] | Pending Litigation [Member] | MacArthur v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																						$ 4,000,000
Number of plaintiffs | plaintiff																						2
Stanford Financial Group [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit																					5
Number of cases removed | lawsuit																				5
Number of cases moved to stay | lawsuit																			4
Number of cases transferred | lawsuit																		5
Claims stayed, period																		7 days
Stanford Financial Group [Member] | Pending Litigation [Member] | Zacarias, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Period to replead dismissed claim																21 days
Stanford Financial Group [Member] | Pending Litigation [Member] | Tisminesky, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Period to replead dismissed claim															21 days
Stanford Financial Group [Member] | Pending Litigation [Member] | de Gadala-Maria, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Period to replead dismissed claim															21 days
Stanford Financial Group [Member] | Pending Litigation [Member] | Martin v. Willis of Colorado, Inc., et. al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)												$ 1,000,000	$ 100,000
Number of plaintiffs | plaintiff												5	1
Stanford Financial Group [Member] | Pending Litigation [Member] | Abel, et al. v. Willis of Colorado, Inc., et al [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)												$ 135,000,000
Number of plaintiffs | plaintiff												300
Stanford Financial Group [Member] | Settled Litigation [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit																														15
Provision for litigation losses																												$ 50,000,000	$ 70,000,000
Litigation settlement amount														$ 120,000,000														$ 120,000,000
Stanford Financial Group [Member] | Settled Litigation [Member] | Troice, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																							$ 1,000,000,000
Number of actions consolidated | action																															2
Stanford Financial Group [Member] | Settled Litigation [Member] | Janvey, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																	$ 1,000,000,000
Total losses incurred by plaintiff																	$ 4,600,000,000
Barbar, et al. v. Willis Group Holdings Public Limited Company, et al. [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																					$ 30,000,000
Number of plaintiffs | plaintiff																					35
de Gadala-Maria, et al. v. Willis Group Holdings Public Limited Company, et al. [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																					$ 83,500,000
Number of plaintiffs | plaintiff																					64
Ranni, et ano. v. Willis Group Holdings Public Limited Company, et al. [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																					$ 3,000,000
Number of plaintiffs | plaintiff																					2
Tisminesky, et al. v. Willis Group Holdings Public Limited Company, et al. [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																					$ 6,500,000
Number of plaintiffs | plaintiff																					11
Zacarias, et al. v. Willis Group Holdings Public Limited Company, et al. [Member] | Pending Litigation [Member] | Florida Suits [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																					$ 12,500,000
Number of plaintiffs | plaintiff																					10
Aon plc [Member] | Pending Litigation [Member] | Stein v. Willis Towers Watson Public Limited Company, et al [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit						1
Aon plc [Member] | Pending Litigation [Member] | Kent v. Willis Towers Watson Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit					1
Aon plc [Member] | Pending Litigation [Member] | Carter v. Willis Towers Watson Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit				1
Aon plc [Member] | Pending Litigation [Member] | Tang v. Willis Towers Watson Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit			1
Aon plc [Member] | Pending Litigation [Member] | Kuznik v. Willis Towers Watson Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of complaints filed | lawsuit	1
Lead Plaintiff [Member] | Pending Litigation [Member] | Willis Towers Watson Merger-Related Securities Litigation [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)		$ 456,000,000