Segment Information - Revenue (Net of Reimbursable Expenses) of the Reported Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 1,859	$ 1,852	$ 6,141	$ 6,124
Income from operations	17	(26)	339	436
HCB [Member]
Segment Reporting Information [Line Items]
Revenues	792		2,435
CRB [Member]
Segment Reporting Information [Line Items]
Revenues	622		2,036
IRR [Member]
Segment Reporting Information [Line Items]
Revenues	319		1,281
BDA [Member]
Segment Reporting Information [Line Items]
Revenues	128		373
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	1,844	1,835	6,070	6,050
Income from operations	256	259	1,107	1,323
Operating Segments [Member] | HCB [Member]
Segment Reporting Information [Line Items]
Revenues	778	733	2,390	2,408
Income from operations	194	143	536	610
Operating Segments [Member] | CRB [Member]
Segment Reporting Information [Line Items]
Revenues	622	600	2,036	1,916
Income from operations	66	48	288	269
Operating Segments [Member] | IRR [Member]
Segment Reporting Information [Line Items]
Revenues	317	321	1,276	1,186
Income from operations	29	32	379	335
Operating Segments [Member] | BDA [Member]
Segment Reporting Information [Line Items]
Revenues	127	181	368	540
Income from operations	$ (33)	$ 36	$ (96)	$ 109