Goodwill and Other Intangible Assets - Components of Goodwill (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	$ 11,011
Accumulated impairment losses, beginning balance	(492)
Goodwill, net, beginning balance	10,519
Goodwill reassigned in segment realignment	0
Goodwill acquired during the period	5
Goodwill disposed of during the period	(5)
Foreign exchange	(62)
Goodwill, gross, ending balance	10,949
Accumulated impairment losses, ending balance	(492)
Goodwill, net, ending balance	10,457
HCB [Member]
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	4,342
Accumulated impairment losses, beginning balance	(130)
Goodwill, net, beginning balance	4,212
Goodwill reassigned in segment realignment	0
Goodwill acquired during the period	0
Goodwill disposed of during the period	0
Foreign exchange	(31)
Goodwill, gross, ending balance	4,311
Accumulated impairment losses, ending balance	(130)
Goodwill, net, ending balance	4,181
CRB [Member]
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	2,261
Accumulated impairment losses, beginning balance	(362)
Goodwill, net, beginning balance	1,899
Goodwill reassigned in segment realignment	72	[1]
Goodwill acquired during the period	0
Goodwill disposed of during the period	0
Foreign exchange	(25)
Goodwill, gross, ending balance	2,308
Accumulated impairment losses, ending balance	(362)
Goodwill, net, ending balance	1,946
IRR [Member]
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	1,851
Accumulated impairment losses, beginning balance	0
Goodwill, net, beginning balance	1,851
Goodwill reassigned in segment realignment	(72)	[1]
Goodwill acquired during the period	5
Goodwill disposed of during the period	(5)
Foreign exchange	(6)
Goodwill, gross, ending balance	1,773
Accumulated impairment losses, ending balance	0
Goodwill, net, ending balance	1,773
BDA [Member]
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	2,557
Accumulated impairment losses, beginning balance	0
Goodwill, net, beginning balance	2,557
Goodwill reassigned in segment realignment	0
Goodwill acquired during the period	0
Goodwill disposed of during the period	0
Foreign exchange	0
Goodwill, gross, ending balance	2,557
Accumulated impairment losses, ending balance	0
Goodwill, net, ending balance	$ 2,557

[1] Represents the reallocation of goodwill related to certain businesses which were realigned among the segments as of January 1, 2018. See Note 4 — Segment Information for further information.