Significant Accounting Policies - Adoption of New Guidance Impact on Previously Reported Quarterly Results (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase in stock-based compensation expense			$ 85	$ 72	$ 63
Reclassification of cash flows related to excess tax benefits from financing activities to operating activities			$ 321	418	407
New Accounting Pronouncement, Early Adoption | ASU 2016-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase in stock-based compensation expense	$ 1
Reclassification of cash flows related to excess tax benefits from financing activities to operating activities				$ 36	$ 20
New Accounting Pronouncement, Early Adoption | ASU 2016-09 | Reclassification from Financing Activities to Operating Activities
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of cash flows related to excess tax benefits from financing activities to operating activities	$ 2	$ 4