Segment Information - Revenue (Net of Reimbursable Expenses) of the Reported Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 2,113	$ 2,048	$ 4,579	$ 4,360
Income from operations	163	176	523	535
HCB [Member]
Segment Reporting Information [Line Items]
Revenues	778	812	1,643	1,655
CRB [Member]
Segment Reporting Information [Line Items]
Revenues	702	690	1,441	1,418
IRR [Member]
Segment Reporting Information [Line Items]
Revenues	414	411	1,032	1,002
BDA [Member]
Segment Reporting Information [Line Items]
Revenues	211	128	445	266
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	2,090	2,022	4,525	4,303
Income from operations	405	357	1,011	919
Operating Segments [Member] | HCB [Member]
Segment Reporting Information [Line Items]
Revenues	767	797	1,617	1,626
Income from operations	160	169	373	373
Operating Segments [Member] | CRB [Member]
Segment Reporting Information [Line Items]
Revenues	701	690	1,440	1,418
Income from operations	135	104	262	231
Operating Segments [Member] | IRR [Member]
Segment Reporting Information [Line Items]
Revenues	413	409	1,028	998
Income from operations	119	109	396	361
Operating Segments [Member] | BDA [Member]
Segment Reporting Information [Line Items]
Revenues	209	126	440	261
Income from operations	$ (9)	$ (25)	$ (20)	$ (46)