Segment Reporting - Segment Revenue and Segment Profit (Loss) (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) primaryreportablesegment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Reportable segments, number \| primaryreportablesegment	3
Total Revenues	$ 5,393	$ 6,022	$ 6,408
Revenue from Contract with Customer, Excluding Assessed Tax	5,393
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	221	254	333
Segment Adjusted EBITDA	640	672	635
Segment profit (loss)	422	417	196
Commercial Industries [Member]
Segment Reporting Information [Line Items]
Total Revenues	2,547	2,685	2,827
Revenue from Contract with Customer, Excluding Assessed Tax	2,547
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	97	98	103
Segment Adjusted EBITDA	597	661	623
Segment profit (loss)	500	563	520
Government services [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,351	1,433	1,575
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	30	41	43
Segment Adjusted EBITDA	451	440	464
Segment profit (loss)	424	398	421
Transportation Services [Member]
Segment Reporting Information [Line Items]
Total Revenues	729	767	766
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	36	43	41
Segment Adjusted EBITDA	149	157	129
Segment profit (loss)	113	114	88
Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	766
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Revenues	0	0	0
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	48	57	70
Segment Adjusted EBITDA	(647)	(745)	(780)
Segment profit (loss)	(695)	(802)	(850)
Disposed of by Sale \| Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	752	1,062	1,109
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	7	13	24
Segment Adjusted EBITDA	105	141	190
Segment profit (loss)	98	128	166
Education [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	14	75	131
Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets	3	2	52
Segment Adjusted EBITDA	(15)	18	9
Segment profit (loss)	$ (18)	$ 16	$ (149)

Depreciation and Amortization, excluding Amortization of Acquired Intangible Assets

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- Definition

Segment Adjusted EBITDA

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- Definition

Amount of profit or loss recognized by the reportable segment.

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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