Summary of Significant Accounting Policies - Weighted Average Number of Common and Common Equivalent Shares Used in Calculation of Basic and Diluted Earnings Per Share (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average common shares outstanding (used in the calculation of basic earnings per share) (in shares)	786	792	795
Potential dilution from stock options and restricted stock and stock units (in shares)	0	0	0
Weighted average common and common equivalent shares (used in the calculation of diluted earnings per share) (in shares)	786	792	795
Antidilutive stock options excluded from the calculation of diluted earnings per share (in shares)	2	6	7