Segment Reporting - Segment Revenue and Segment Profit (Loss) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) primaryreportablesegment	Jun. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Primary reportable segments | primaryreportablesegment			3
Revenues	$ 1,016	$ 1,112	$ 2,067	$ 2,270
Segment Pre-tax Income (Loss)	55	59	100	128
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	55	51	113	105
Segment Adjusted EBITDA	110	114	206	237
Commercial Industries
Segment Reporting Information [Line Items]
Revenues	520	592	1,092	1,204
Segment Pre-tax Income (Loss)	72	111	162	228
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	24	21	49	43
Segment Adjusted EBITDA	96	132	211	271
Government services
Segment Reporting Information [Line Items]
Revenues	331	326	621	651
Segment Pre-tax Income (Loss)	115	95	208	175
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	5	7	11	16
Segment Adjusted EBITDA	120	102	219	191
Transportation Services
Segment Reporting Information [Line Items]
Revenues	165	194	354	378
Segment Pre-tax Income (Loss)	31	29	54	48
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	8	8	17	17
Segment Adjusted EBITDA	39	41	71	69
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Segment Pre-tax Income (Loss)	(162)	(176)	(327)	(324)
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	18	15	36	29
Segment Adjusted EBITDA	(144)	(161)	(291)	(295)
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)	(1)	0	3	0
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	0	0	0	0
Segment Adjusted EBITDA	$ (1)	$ 0	$ (4)	$ 0