Quarterly Earnings Summary (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Earnings Summary (Unaudited) [Abstract]
Interest income	$ 270	$ 273	$ 281	$ 306	$ 306	$ 314	$ 317	$ 310	$ 1,130	$ 1,247	$ 1,170
Interest expense	36	46	52	74	80	84	87	79	208	330	238
Net Interest Income	234	227	229	232	226	230	230	231	922	917	932
Provision for credit losses	18	27	30	48	7	12	11	14	123	44	61
Total non-interest income	68	80	77	69	74	80	75	65	294	294	276
Total non-interest expense	199	180	176	195	177	178	175	166	750	696	695
Net income	72	83	84	47	95	103	95	94	286	387	373
Net income available to common stockholders	$ 70	$ 81	$ 82	$ 45	$ 93	$ 101	$ 93	$ 92	$ 278	$ 379	$ 365
Earnings per Common Share
Basic (in usd per share)	$ 0.22	$ 0.25	$ 0.25	$ 0.14	$ 0.29	$ 0.31	$ 0.29	$ 0.28	$ 0.86	$ 1.17	$ 1.13
Diluted (in usd per share)	$ 0.22	$ 0.25	$ 0.25	$ 0.14	$ 0.29	$ 0.31	$ 0.29	$ 0.28	$ 0.85	$ 1.16	$ 1.12