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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 95	$ 103	$ 95	$ 94	$ 100	$ 101	$ 85	$ 87	$ 387	$ 373	$ 199
Less: Preferred stock dividends									8	8	8
Net Income Available to Common Stockholders	$ 93	$ 101	$ 93	$ 92	$ 98	$ 99	$ 83	$ 85	$ 379	$ 365	$ 191
Basic weighted average common shares outstanding (in shares)									324,938,720	324,207,198	302,195,295
Net effect of dilutive stock options, warrants and restricted stock (in shares)									1,122,418	1,416,405	1,662,681
Diluted weighted average common shares outstanding (in shares)									326,061,138	325,623,603	303,857,976
Earnings per common share:
Basic (in usd per share)	$ 0.29	$ 0.31	$ 0.29	$ 0.28	$ 0.30	$ 0.30	$ 0.26	$ 0.26	$ 1.17	$ 1.13	$ 0.63
Diluted (in usd per share)	$ 0.29	$ 0.31	$ 0.29	$ 0.28	$ 0.30	$ 0.30	$ 0.26	$ 0.26	$ 1.16	$ 1.12	$ 0.63