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Accounting Policies - Narrative (Details) - USD ($)
shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2016
Dec. 31, 2014
Significant Accounting Policies [Line Items]          
Net unrealized gain on equity securities included in AOCI $ 840 $ 404      
Impact of the U.S. corporate tax rate change on AOCI 0       $ 2
Tax benefit related to exercise of stock options $ 18 $ 11 $ 16    
Weighted average common shares adjustment related to stock-based compensation 2.0 1.6 1.8    
Anti-dilutive potential common shares related to stock-based compensation plans 0.0 0.4 1.1    
Maturities of short term investments 3 months        
Accounting Standards Update 2014-09          
Significant Accounting Policies [Line Items]          
Expected effect on consolidated revenues under new revenue recognition guidance effective in 2018 2.00%        
Retained Earnings          
Significant Accounting Policies [Line Items]          
Impact of the U.S. corporate tax rate change on AOCI $ (145)       $ 2
Retained Earnings | Maximum          
Significant Accounting Policies [Line Items]          
Impact of the U.S. corporate tax rate change on AOCI       $ 1  
Stock Options | Capital surplus under previous accounting guidance          
Significant Accounting Policies [Line Items]          
Tax benefit related to exercise of stock options 17 $ 9      
Equity securities          
Significant Accounting Policies [Line Items]          
Net unrealized gain on equity securities included in AOCI [1] 221 $ 98      
Equity securities | Accounting Standards Update 2016-01          
Significant Accounting Policies [Line Items]          
Net unrealized gain on equity securities included in AOCI 221        
Tax Cuts and Jobs Act of 2017 | Accounting Standards Update 2018-02          
Significant Accounting Policies [Line Items]          
Impact of the U.S. corporate tax rate change on AOCI $ 145        
[1] As discussed in Note A — “Accounting Policies — Investments,” effective January 1, 2018, all equity securities currently classified as “available for sale” will be required to be carried at fair value through net earnings instead of AOCI.