Stockholders' Equity - Shares Repurchased (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Number of shares repurchased	7,720,194	2,317,617	3,979,170
Average price per share (in dollars per share)	$ 117.02	$ 127.04	$ 109.64
Total cost of repurchases	$ 903	$ 294	$ 436

Number of shares that have been repurchased during the period and are being held in treasury.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration