Revenue Recognition - Impacts of ASC 606 on Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 673	$ 325	$ 325	$ 300
Trade accounts receivable, net	949	997
Prepaid expenses and other current assets	716	603
Total current assets	2,338	1,975
Property and equipment, net	385	390
Intangible assets, net	1,802	1,882
Goodwill	5,450	5,590
Contract costs, net	410	84
Other long-term assets	363	368
Total assets	10,748	10,289
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	1,551	1,359
Current maturities of long-term debt	452	3
Contract liabilities	317	576
Total current liabilities	2,320	1,938
Long-term debt	4,823	4,897
Deferred income taxes	715	552
Long-term contract liabilities	75	54
Other long-term liabilities	154	117
Total liabilities	8,087	7,558
Total shareholders’ equity	2,661	2,731
Total liabilities and shareholders’ equity	10,748	10,289
Adjustments | Accounting Standards Update 2014-09
Assets
Cash and cash equivalents	0	0
Trade accounts receivable, net	(20)
Prepaid expenses and other current assets	26
Total current assets	6
Property and equipment, net	0
Intangible assets, net	0
Goodwill	0
Contract costs, net	(327)
Other long-term assets	98
Total assets	(223)
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	(9)
Current maturities of long-term debt	0
Contract liabilities	95
Total current liabilities	86
Long-term debt	0
Deferred income taxes	(74)
Long-term contract liabilities	21
Other long-term liabilities	0
Total liabilities	33
Total shareholders’ equity	(256)
Total liabilities and shareholders’ equity	(223)
Balances without adoption of ASC 606
Assets
Cash and cash equivalents	673	$ 325
Trade accounts receivable, net	929
Prepaid expenses and other current assets	742
Total current assets	2,344
Property and equipment, net	385
Intangible assets, net	1,802
Goodwill	5,450
Contract costs, net	83
Other long-term assets	461
Total assets	10,525
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	1,542
Current maturities of long-term debt	452
Contract liabilities	412
Total current liabilities	2,406
Long-term debt	4,823
Deferred income taxes	641
Long-term contract liabilities	96
Other long-term liabilities	154
Total liabilities	8,120
Total shareholders’ equity	2,405
Total liabilities and shareholders’ equity	$ 10,525