Segment Reporting - Segment Revenue and Segment Profit (Loss) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) primaryreportablesegment	Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Primary reportable segments | primaryreportablesegment	3
Revenues	$ 1,051	$ 1,158
Segment Pre-tax Income (Loss)	45	69
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	58	54
Segment Adjusted EBITDA	96	123
Commercial Industries
Segment Reporting Information [Line Items]
Revenues	572	612
Segment Pre-tax Income (Loss)	90	117
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	25	22
Segment Adjusted EBITDA	115	139
Government services
Segment Reporting Information [Line Items]
Revenues	290	325
Segment Pre-tax Income (Loss)	93	80
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	6	9
Segment Adjusted EBITDA	99	89
Transportation Services
Segment Reporting Information [Line Items]
Revenues	189	184
Segment Pre-tax Income (Loss)	23	19
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	9	9
Segment Adjusted EBITDA	32	28
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Segment Pre-tax Income (Loss)	(165)	(148)
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	18	14
Segment Adjusted EBITDA	(147)	(134)
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Other
Segment Reporting Information [Line Items]
Revenues	0	36
Segment Pre-tax Income (Loss)	0	1
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	0	0
Segment Adjusted EBITDA	0	1
Education | Other
Segment Reporting Information [Line Items]
Revenues	0	1
Segment Pre-tax Income (Loss)	4	0
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	0	0
Segment Adjusted EBITDA	$ (3)	$ 0