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Summary of Significant Accounting Policies (Accounting Policies) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Accounting Change [Line Items]        
Provision for income taxes $ 16 $ (177)    
Variable Interest Entity Nonconsolidated Carrying Amount Assets And Liabilities Net     $ 6,110  
Net Assets And Noncontrolling Interest Decreases From Deconsolidation       $ 123
Deferred Compensation, Share-based Payments [Members]        
Accounting Change [Line Items]        
Provision for income taxes (41)   (97)  
Excess Tax Benefit From Share Based Compensation Operating Activities (59)   (137)  
Excess Tax Benefit From Share Based Compensation Financing Activities $ (59)   $ (137)