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Claims, Lawsuits and Other Contingencies - Narrative (Details) £ in Millions, $ in Millions						3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Sep. 06, 2018 USD ($)	Oct. 03, 2017 NZD ($)	Dec. 20, 2016 GBP (£)	Jun. 29, 2015 NZD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Apr. 30, 2014 GBP (£)
Legal, Guarantees and Indemnifications
Revenue						$ 3,143,000,000	$ 3,090,000,000
Maximum potential funding under commitments	$ 76,000,000					76,000,000			$ 103,000,000
Letters of credit outstanding	92,000,000					92,000,000			$ 83,000,000
Potential Claim for Pension Advisory Services
Legal, Guarantees and Indemnifications
Estimate of possible loss	59,000,000					59,000,000				£ 45
Pending Litigation \| Trustees Of Gleeds Pension Fund 2016
Legal, Guarantees and Indemnifications
Estimate of possible loss	92,000,000					92,000,000
Damages sought \| £				£ 70
Pending Litigation \| Lyttleton Port Company Limited
Legal, Guarantees and Indemnifications
Damages sought	127,000,000				$ 184
Pending Litigation \| Christchurch City Council
Legal, Guarantees and Indemnifications
Damages sought	365,000,000		$ 528
Minimum
Legal, Guarantees and Indemnifications
Estimate of possible loss	0					0
Minimum \| Pilkington North America, Inc.
Legal, Guarantees and Indemnifications
Damages sought		$ 45,000,000
Maximum
Legal, Guarantees and Indemnifications
Estimate of possible loss	100,000,000					100,000,000
Maximum \| Pilkington North America, Inc.
Legal, Guarantees and Indemnifications
Damages sought		85,000,000
Damages awarded		$ 15,000,000
Aviation and Aerospace Broking Industry
Legal, Guarantees and Indemnifications
Revenue								$ 100,000,000
Discontinued Operations, Disposed of by Sale \| Tempo Business \| Property Lease Guarantee
Legal, Guarantees and Indemnifications
Guarantor obligations, current carrying value	16,000,000					16,000,000
Loss contingency accrual payments						0
Maximum potential funding under commitments	81,000,000					81,000,000
Discontinued Operations, Disposed of by Sale \| Tempo Business \| Performance Guarantee
Legal, Guarantees and Indemnifications
Guarantor obligations, current carrying value	1,000,000					1,000,000
Loss contingency accrual payments						0
Maximum potential funding under commitments	$ 179,000,000					$ 179,000,000