Revenue Recognition - Impacts of ASC 606 on Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 348	$ 325	$ 299	$ 300
Trade accounts receivable, net	932	997
Prepaid expenses and other current assets	524	603
Total current assets	1,804	1,975
Property and equipment, net	374	390
Intangible assets, net	1,833	1,882
Goodwill	5,456	5,590
Contract costs, net	398	84
Other long-term assets	353	368
Total assets	10,218	10,289
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	1,302	1,359
Current maturities of long-term debt	1	3
Contract liabilities	352	576
Total current liabilities	1,655	1,938
Long-term debt	4,805	4,897
Deferred income taxes	692	552
Long-term contract liabilities	71	54
Other long-term liabilities	145	117
Total liabilities	7,368	7,558
Total shareholders’ equity	2,850	2,731
Total liabilities and shareholders’ equity	10,218	10,289
Adjustments | Accounting Standards Update 2014-09
Assets
Cash and cash equivalents	0	0
Trade accounts receivable, net	(9)
Prepaid expenses and other current assets	17
Total current assets	8
Property and equipment, net	0
Intangible assets, net	0
Goodwill	0
Contract costs, net	(319)
Other long-term assets	94
Total assets	(217)
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	(17)
Current maturities of long-term debt	0
Contract liabilities	98
Total current liabilities	81
Long-term debt	0
Deferred income taxes	(72)
Long-term contract liabilities	20
Other long-term liabilities	0
Total liabilities	29
Total shareholders’ equity	(246)
Total liabilities and shareholders’ equity	(217)
Balances without adoption of ASC 606
Assets
Cash and cash equivalents	348	$ 325
Trade accounts receivable, net	923
Prepaid expenses and other current assets	541
Total current assets	1,812
Property and equipment, net	374
Intangible assets, net	1,833
Goodwill	5,456
Contract costs, net	79
Other long-term assets	447
Total assets	10,001
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	1,285
Current maturities of long-term debt	1
Contract liabilities	450
Total current liabilities	1,736
Long-term debt	4,805
Deferred income taxes	620
Long-term contract liabilities	91
Other long-term liabilities	145
Total liabilities	7,397
Total shareholders’ equity	2,604
Total liabilities and shareholders’ equity	$ 10,001