Net Earnings Per Common Share - Basic and diluted net earnings per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic:
Net income	$ 66	$ 198	$ 80	$ 411
Less common and preferred dividends	66	63	130	127
Undistributed earnings	0	135	(50)	284
Less undistributed earnings applicable to nonvested shares	0	1	0	2
Undistributed earnings applicable to common shares	0	134	(50)	282
Distributed earnings applicable to common shares	56	54	112	110
Total earnings applicable to common shares	$ 56	$ 188	$ 62	$ 392
Weighted average common shares outstanding (in shares)	163,542	179,156	163,843	181,946
Net earnings per common share (in dollars per share)	$ 0.34	$ 1.05	$ 0.38	$ 2.15
Diluted:
Dilutive effect of common stock warrants (in shares)	723	9,318	4,012	9,587
Dilutive effect of stock options (in shares)	160	624	277	673
Weighted average diluted common shares outstanding (in shares)	164,425	189,098	168,132	192,206
Net earnings per common share (in dollars per share)	$ 0.34	$ 0.99	$ 0.37	$ 2.04