Reconciliation of Previously Reported Amounts to Recast Financial Statements - Reconciliation of Amounts Previously Reported (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Costs of services (excludes depreciation and amortization and restructuring costs)	$ 3,725		$ 4,051				$ 11,110	$ 12,230
Selling, general, and administrative (excludes depreciation and amortization and restructuring costs)	491		447				1,500	1,484
Other income, net	(145)		(75)				(336)	(291)
Decrease (increase) in receivables									$ (464)	$ (25)
Decrease (increase) in assets		$ (300)		$ (182)	$ (483)	$ (261)	(1,012)	(365)
Net cash provided by operating activities		369		379	849	1,204	1,035	2,010	2,567	619
Cash collections related to deferred purchase price receivable		137		155	445	339	761	531	685	359
Other investing activities, net						6	9	20
Net cash provided by investing activities		(180)		1,014	(48)	802	(40)	770	719	(565)
Cash and cash equivalents at beginning of year		2,729	2,702	1,268	2,729	1,268	2,729	1,268	1,268
Cash and cash equivalents at end of period	$ 2,475		2,956			2,702	$ 2,475	2,956	2,729	1,268
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Costs of services (excludes depreciation and amortization and restructuring costs)			4,521					13,621
Selling, general, and administrative (excludes depreciation and amortization and restructuring costs)			475					1,557
Other income, net			8					(72)
Decrease (increase) in receivables									74	193
Decrease (increase) in assets		(196)		(27)	(447)	78		167
Net cash provided by operating activities		473		534	885	1,543		2,542	3,105	837
Cash collections related to deferred purchase price receivable		33		0	409	0		0	147	141
Other investing activities, net						(20)		(6)
Net cash provided by investing activities		(284)		859	(84)	437		213	181	(783)
Cash and cash equivalents at beginning of year			2,671	1,263		1,263		1,263	1,263
Cash and cash equivalents at end of period			2,926			2,671		2,926		1,263
Restatement Adjustment | Reclassification of Discontinued Operations
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Costs of services (excludes depreciation and amortization and restructuring costs)			(544)					(1,589)
Selling, general, and administrative (excludes depreciation and amortization and restructuring costs)			(37)					(94)
Other income, net			0					0
Accounting Standards Update 2017-07 | Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Costs of services (excludes depreciation and amortization and restructuring costs)			74					198
Selling, general, and administrative (excludes depreciation and amortization and restructuring costs)			9					21
Other income, net			(83)					(219)
Accounting Standards Update 2016-15 | Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease (increase) in receivables									(538)	(218)
Decrease (increase) in assets		(104)		(155)	(36)	(339)		(531)
Net cash provided by operating activities		(104)		(155)	(36)	(339)		(531)	(538)	(218)
Cash collections related to deferred purchase price receivable		104		155	36	339		531	538	218
Other investing activities, net						0		0
Net cash provided by investing activities		$ 104		155	$ 36	339		531	538	218
Cash and cash equivalents at beginning of year			0	0		0		0	0
Cash and cash equivalents at end of period			0			0		0		0
Accounting Standards Update 2016-18 | Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease (increase) in assets						0		(1)
Net cash provided by operating activities						0		(1)
Cash collections related to deferred purchase price receivable						0		0
Other investing activities, net						26		26
Net cash provided by investing activities						26		26
Cash and cash equivalents at beginning of year			31	$ 5		5		5	$ 5
Cash and cash equivalents at end of period			$ 30			$ 31		$ 30		$ 5