Commitments and Contingencies - Narrative (Details)																	3 Months Ended		86 Months Ended
	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
Settled Litigation [Member] | Stanford Financial Group [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
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
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
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
Pending Litigation [Member] | Rupert, et al. v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff															97
Damages sought (in excess of)															$ 300,000,000
Pending Litigation [Member] | Casanova, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff														7
Damages sought (in excess of)														$ 5,000,000
Pending Litigation [Member] | Rishmague, et ano. v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff													2
Damages sought (in excess of)													$ 37,000,000
Pending Litigation [Member] | MacArthur v. Winter, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff											2
Damages sought (in excess of)											$ 4,000,000
Pending Litigation [Member] | Stanford Financial Group, 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
Pending Litigation [Member] | Barbar, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff										35
Damages sought (in excess of)										$ 30,000,000
Pending Litigation [Member] | de Gadala-Maria, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff										64
Damages sought (in excess of)										$ 83,500,000
Period to replead dismissed claim				21 days
Pending Litigation [Member] | Ranni, et ano. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff										2
Damages sought (in excess of)										$ 3,000,000
Pending Litigation [Member] | Tisminesky, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff										11
Damages sought (in excess of)										$ 6,500,000
Period to replead dismissed claim				21 days
Pending Litigation [Member] | Zacarias, et al. v. Willis Group Holdings Public Limited Company, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff										10
Damages sought (in excess of)										$ 12,500,000
Period to replead dismissed claim					21 days
Pending Litigation [Member] | Martin v. Willis of Colorado, Inc., et. al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff	5	1
Damages sought (in excess of)	$ 1,000,000	$ 100,000
Pending Litigation [Member] | Abel, et al. v. Willis of Colorado, Inc., et al [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff	300
Damages sought (in excess of)	$ 135,000,000