Earnings Per Common Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) - 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
Earnings Per Share [Abstract]
Net income	$ 72	$ 83	$ 84	$ 47	$ 95	$ 103	$ 95	$ 94	$ 286	$ 387	$ 373
Less: Preferred stock dividends									8	8	8
Net Income Available to Common Stockholders	$ 70	$ 81	$ 82	$ 45	$ 93	$ 101	$ 93	$ 92	$ 278	$ 379	$ 365
Basic weighted average common shares outstanding (in shares)									323,368,639	324,938,720	324,207,198
Net effect of dilutive stock options, warrants and restricted stock (in shares)									2,119,325	1,122,418	1,416,405
Diluted weighted average common shares outstanding (in shares)									325,487,964	326,061,138	325,623,603
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