Claims, Lawsuits, and Other Contingencies - Legal Narrative (Details) £ in Millions, $ in Millions				1 Months Ended					12 Months Ended
	Dec. 31, 2021 USD ($)	Sep. 06, 2018 USD ($)	Oct. 03, 2017 NZD ($)	Apr. 30, 2021 USD ($) plaintiff	Apr. 30, 2021 GBP (£) plaintiff	Aug. 31, 2020 USD ($) plaintiff	Nov. 30, 2019 USD ($) defendant	Nov. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)	Sep. 16, 2020 USD ($)	Apr. 27, 2018 USD ($)
Aviation and Aerospace Broking Industry
Loss Contingencies [Line Items]
Total revenue									$ 100,000,000
Aon Hewitt Investment Consulting, Inc.
Loss Contingencies [Line Items]
Investment losses suffered by plaintiff											$ 200,000,000
Pending Litigation | Christchurch City Council
Loss Contingencies [Line Items]
Value of damages sought	$ 218,000,000		$ 320
Pending Litigation | Fatal Plain Crash In November2016
Loss Contingencies [Line Items]
Value of damages sought				$ 39,000,000	£ 29	$ 844,000,000		$ 16,000,000
Loss contingency, number of defendants | defendant							3
Damages sought option 1							$ 300,000,000
Damages sought option 2							50,000,000
Damages sought option 3							$ 25,000,000
Loss contingency, number of plaintiffs | plaintiff				16	16	43
Pending Litigation | N E B C
Loss Contingencies [Line Items]
Investment losses suffered by plaintiff										$ 2,000,000,000
Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	0
Minimum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought		$ 45,000,000
Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	$ 800,000,000
Maximum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought		85,000,000
Damages awarded		$ 15,000,000