Segment Reporting - Segment Revenue and Segment Profit (Loss) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) primaryreportablesegment	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Primary reportable segments | primaryreportablesegment			3
Revenues	$ 1,041	$ 1,098	$ 3,108	$ 3,368
Segment Pre-tax Income (Loss)	88	72	188	200
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	53	55	166	160
Segment Adjusted EBITDA	141	127	347	364
Commercial Industries
Segment Reporting Information [Line Items]
Revenues	518	578	1,610	1,782
Segment Pre-tax Income (Loss)	32	60	99	198
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	24	27	79	76
Segment Adjusted EBITDA	56	87	178	274
Government services
Segment Reporting Information [Line Items]
Revenues	348	319	969	970
Segment Pre-tax Income (Loss)	122	70	282	202
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	6	7	19	24
Segment Adjusted EBITDA	128	77	301	226
Transportation Services
Segment Reporting Information [Line Items]
Revenues	175	201	529	579
Segment Pre-tax Income (Loss)	26	25	56	50
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	9	9	27	27
Segment Adjusted EBITDA	35	34	83	81
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Segment Pre-tax Income (Loss)	(98)	(83)	(258)	(251)
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	14	12	41	33
Segment Adjusted EBITDA	(84)	(71)	(217)	(218)
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Other
Segment Reporting Information [Line Items]
Revenues	0	0	0	36
Segment Pre-tax Income (Loss)	0	0	0	1
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	0	0	0	0
Segment Adjusted EBITDA	0	0	0	1
Education | Other
Segment Reporting Information [Line Items]
Revenues	0	0	0	1
Segment Pre-tax Income (Loss)	6	0	9	0
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	0	0	0	0
Segment Adjusted EBITDA	$ 6	$ 0	$ 2	$ 0