Parent Company Financial Statements - Consolidated Balance Sheet of Parent Company Only (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 488	$ 479	$ 371	$ 489
Debt securities available for sale	3,341
Other assets	823	810
Total Assets	33,102	31,418	21,845
Other liabilities	283	262
Long-term borrowings	627	668
Long-term	87	88
Total Liabilities	28,494	27,009
Stockholders' Equity	4,608	4,409	2,572	2,096
Total Liabilities and Stockholders’ Equity	33,102	31,418
Parent Company
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	254	166	$ 164	$ 228
Debt securities available for sale	0	1
Other assets	19	22
Total Assets	5,124	5,037
Other liabilities	32	33
Advances from affiliates	197	306
Long-term borrowings	279	280
Short-term	7	8
Long-term	1	1
Total Liabilities	516	628
Stockholders' Equity	4,608	4,409
Total Liabilities and Stockholders’ Equity	5,124	5,037
Parent Company | Bank Subsidiary
Condensed Balance Sheet Statements, Captions [Line Items]
Investment in and advances to subsidiaries	4,754	4,554
Parent Company | Non-bank Subsidiaries
Condensed Balance Sheet Statements, Captions [Line Items]
Investment in and advances to subsidiaries	$ 97	$ 294