Net Income Per Share Attributable To Common Stockholders (Narrative) (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 13, 2018
Equity, Class of Treasury Stock [Line Items]
Basic and diluted weighted average common shares outstanding	165	167	171
Repurchase of common stock	$ 176,000,000	$ 653,000,000	$ 179,000,000
Stock repurchase program
Equity, Class of Treasury Stock [Line Items]
Authorized repurchase amount	2,000,000,000
Repurchase of common stock			127,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	1,100,000,000
Previously Authorized Common Stock Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock			22,000,000
Shares repurchased
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock	$ 160,000,000	$ 622,000,000	$ 149,000,000