Parent Company Financial Statements - Statements of Income of Parent Company Only (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Interest income	$ 305	$ 298	$ 294	$ 273	$ 271	$ 263	$ 251	$ 195	$ 1,170	$ 980	$ 679
Interest income									1	1	0
Interest expense	73	63	55	47	41	38	33	22	238	134	67
Income tax benefit									(79)	(157)	(75)
Net income	100	$ 101	$ 85	$ 87	24	$ 78	$ 74	$ 23	373	199	171
Parent Company
Condensed Income Statements, Captions [Line Items]
Dividend income from subsidiaries:									170	158	118
Interest income									4	5	5
Interest income									5	0	3
Total Income									179	163	126
Interest expense									20	18	14
Other expenses									15	10	10
Total Expenses									35	28	24
Income Before Taxes and Equity in Undistributed Income of Subsidiaries									144	135	102
Income tax benefit									6	3	6
Income after income taxes									150	138	108
Equity in undistributed income (loss) of subsidiaries:	222				61				222	61	63
Net income									373	199	171
Parent Company | Bank
Condensed Income Statements, Captions [Line Items]
Dividend income from subsidiaries:									162	149	109
Equity in undistributed income (loss) of subsidiaries:	225				60				225	60	61
Parent Company | Non-Bank
Condensed Income Statements, Captions [Line Items]
Dividend income from subsidiaries:									8	9	9
Equity in undistributed income (loss) of subsidiaries:	$ (2)				$ 1				$ (2)	$ 1	$ 2