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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic:
Net income	$ 175	$ 222	$ 255	$ 633
Less common and preferred dividends	64	67	193	194
Undistributed earnings	111	155	62	439
Less undistributed earnings applicable to nonvested shares	1	1	1	2
Undistributed earnings applicable to common shares	110	154	61	437
Distributed earnings applicable to common shares	56	59	168	168
Total earnings applicable to common shares	$ 166	$ 213	$ 229	$ 605
Weighted average common shares outstanding (in shares)	163,608	173,160	163,764	178,985
Net earnings per common share (in dollars per share)	$ 1.01	$ 1.23	$ 1.40	$ 3.38
Diluted:
Dilutive effect of common stock warrants (in shares)	0	8,187	2,027	9,293
Dilutive effect of stock options (in shares)	171	523	238	617
Weighted average diluted common shares outstanding (in shares)	163,779	181,870	166,029	188,895
Net earnings per common share (in dollars per share)	$ 1.01	$ 1.17	$ 1.38	$ 3.20