Commitments and Contingencies (Details)																						1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 05, 2018 USD ($)	Mar. 01, 2017 USD ($)	Sep. 12, 2016 USD ($) plaintiff	Aug. 05, 2016 USD ($) plaintiff	Mar. 31, 2016 USD ($)	Jul. 21, 2015	Jul. 15, 2015	Aug. 01, 2014 USD ($)	Jan. 10, 2014 plaintiff	Oct. 01, 2013 USD ($)	Aug. 06, 2013 plaintiff	Jun. 20, 2013 lawsuit	Jun. 11, 2013 lawsuit	Jun. 03, 2013 lawsuit	Feb. 14, 2013 USD ($) lawsuit plaintiff	Feb. 08, 2013 USD ($) plaintiff	Mar. 11, 2011 USD ($) plaintiff	Sep. 16, 2010 USD ($) plaintiff	Sep. 14, 2009 USD ($) plaintiff	Aug. 06, 2009 USD ($)	Jul. 02, 2009 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2018 lawsuit	Jul. 01, 2017 USD ($)	May 08, 2017 USD ($)	Mar. 25, 2014 action
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
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] | Canabal, et al. v. Willis of Colorado, Inc., et al. [Member]
Loss Contingencies [Line Items]
Damages sought (in excess of)																				$ 1,000,000,000
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
Settled 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
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
Settled 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
Settled 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
Settled Litigation [Member] | City of Houston [Member]
Loss Contingencies [Line Items]
Litigation Settlement amount																						$ 40,000,000
Increase to actuarial accrued liability alleged by plaintiff								$ 163,000,000
Estimated damages incurred (through July 1, 2017)																											$ 430,000,000
Estimated future damages incurred (as of July 1, 2017)																										$ 400,000,000
Settled Litigation [Member] | Elma Sanchez, et al. [Member]
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff									3		3
Litigation Settlement amount	$ 9,750,000	$ 9,750,000
Premium rate increase											85.00%