Stockholders' Equity - Impact to Equity as a Result of Stock Repurchases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity, Repurchased Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 2,900	$ 954	$ 11,001
Common Stock Including Additional Paid in Capital [Member]
Equity, Repurchased Stock [Line Items]
Stock Repurchased and Retired During Period, Value	112	34	302
Retained Earnings [Member]
Equity, Repurchased Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 2,940	$ 1,028	$ 10,883