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Claims, Lawsuits and Other Contingencies - Narrative (Details) $ in Millions				3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Sep. 06, 2018 USD ($)	Oct. 03, 2017 NZD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)
Legal, Guarantees and Indemnifications
Revenue				$ 3,219,000,000	$ 3,143,000,000
Maximum potential funding under commitments	$ 70,000,000			70,000,000			$ 110,000,000
Letters of credit outstanding	71,000,000			71,000,000			$ 73,000,000
Pending Litigation \| Christchurch City Council
Legal, Guarantees and Indemnifications
Damages sought	315,000,000		$ 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
Maximum potential funding under commitments	66,000,000			66,000,000
Guarantor obligations, current carrying value	11,000,000			11,000,000
Loss contingency accrual payments				0
Discontinued Operations, Disposed of by Sale \| Tempo Business \| Performance Guarantee
Legal, Guarantees and Indemnifications
Maximum potential funding under commitments	139,000,000			139,000,000
Guarantor obligations, current carrying value	$ 1,000,000			1,000,000
Loss contingency accrual payments				$ 0