Claims, Lawsuits, and Other Contingencies - Legal Narrative (Details) $ in Millions				1 Months Ended			12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 NZD ($)	Sep. 06, 2018 USD ($)	Aug. 31, 2020 USD ($) plaintiff	Nov. 30, 2019 USD ($) numberOfDefendants	Nov. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)	Sep. 16, 2020 USD ($)	Apr. 27, 2018 USD ($)
Aon Hewitt Investment Consulting, Inc.
Loss Contingencies [Line Items]
Investment losses suffered by plaintiff									$ 100,000,000
Pending Litigation | Christchurch City Council
Loss Contingencies [Line Items]
Value of damages sought	$ 376,000,000	$ 528
Pending Litigation | Fatal Plain Crash In November2016
Loss Contingencies [Line Items]
Value of damages sought				$ 844,000,000		$ 15,500,000
Loss contingency, number of defendants | numberOfDefendants					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				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
Aviation and Aerospace Broking Industry
Loss Contingencies [Line Items]
Total revenue							$ 100,000,000