Segment Information - Revenue (Net of Reimbursable Expenses) of the Reported Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 2,048	$ 1,990	$ 4,360	$ 4,282
Income from operations	176	63	535	322
HCB [Member]
Segment Reporting Information [Line Items]
Revenues	812	797	1,655	1,643
CRB [Member]
Segment Reporting Information [Line Items]
Revenues	690	674	1,418	1,414
IRR [Member]
Segment Reporting Information [Line Items]
Revenues	411	386	1,002	962
BDA [Member]
Segment Reporting Information [Line Items]
Revenues	128	121	266	245
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	2,022	1,958	4,303	4,226
Income from operations	357	304	919	851
Operating Segments [Member] | HCB [Member]
Segment Reporting Information [Line Items]
Revenues	797	780	1,626	1,612
Income from operations	169	149	373	342
Operating Segments [Member] | CRB [Member]
Segment Reporting Information [Line Items]
Revenues	690	674	1,418	1,414
Income from operations	104	97	231	222
Operating Segments [Member] | IRR [Member]
Segment Reporting Information [Line Items]
Revenues	409	385	998	959
Income from operations	109	89	361	350
Operating Segments [Member] | BDA [Member]
Segment Reporting Information [Line Items]
Revenues	126	119	261	241
Income from operations	$ (25)	$ (31)	$ (46)	$ (63)