Fixed Assets - Components of Fixed Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022
Movement in Property, Plant and Equipment [Roll Forward]
Cost: at beginning of period		$ 1,987	$ 2,092
Additions		251	198
Acquisitions			1
Disposals	[1]	(216)	(207)
Foreign exchange		49	(97)
Cost: at end of period		2,071	1,987
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Depreciation: at beginning of period		(1,269)	(1,241)
Depreciation expense		(242)	(255)
Disposals		189	170
Foreign exchange		(29)	57
Depreciation: at end of period		(1,351)	(1,269)
Fixed assets, net		720	718
Furniture, equipment and software [Member]
Movement in Property, Plant and Equipment [Roll Forward]
Cost: at beginning of period		1,452	1,477
Additions		219	174
Acquisitions			1
Disposals	[1]	(182)	(129)
Foreign exchange		38	(71)
Cost: at end of period		1,527	1,452
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Depreciation: at beginning of period		(933)	(877)
Depreciation expense		(202)	(211)
Disposals		164	113
Foreign exchange		(23)	42
Depreciation: at end of period		(994)	(933)
Fixed assets, net		533	519
Leasehold Improvements [Member]
Movement in Property, Plant and Equipment [Roll Forward]
Cost: at beginning of period		452	527
Additions		32	24
Acquisitions			0
Disposals	[1]	(34)	(78)
Foreign exchange		9	(21)
Cost: at end of period		459	452
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Depreciation: at beginning of period		(272)	(301)
Depreciation expense		(37)	(40)
Disposals		25	57
Foreign exchange		(5)	12
Depreciation: at end of period		(289)	(272)
Fixed assets, net		170	180
Land and Building [Member]
Movement in Property, Plant and Equipment [Roll Forward]
Cost: at beginning of period		83	88
Additions		0	0
Acquisitions			0
Disposals	[1]	0	0
Foreign exchange		2	(5)
Cost: at end of period		85	83
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment [Roll Forward]
Depreciation: at beginning of period		(64)	(63)
Depreciation expense		(3)	(4)
Disposals		0	0
Foreign exchange		(1)	3
Depreciation: at end of period		(68)	(64)
Fixed assets, net		$ 17	$ 19

[1] For 2023 and 2022, includes $17 million and $12 million, respectively, of furniture, equipment and software costs and $4 million and $18 million, respectively, of leasehold improvements costs which have been written off as part of technology modernization and real estate rationalization, respectively, under the Transformation program (see Note 6 – Restructuring Costs).