Income (Loss) and Dividends per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	6			5			5	6
Net income	$ 165	$ 161	$ 174	$ 254	$ 96	$ 356	$ 500	$ 706
Weighted average common shares outstanding (in shares)	802			848			813	852
Effect of dilutive securities (in shares)	1			1			0	1
Weighted average common shares, diluted (in shares)	803			849			813	853
Net income per share:
Basic	$ 0.21			$ 0.30			$ 0.62	$ 0.83
Per diluted share:
Diluted	0.21			0.30			0.62	0.83
Dividends per share (in dollars per share)	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.15	$ 0.15