Securities - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Securities [Line Items]
Available for sale, due in one year or less, amortized cost	$ 95
Available for sale, due after one year but within five years, amortized cost	239
Available for sale, due after five year but within ten years, amortized cost	68
Available for sale, due after ten years, amortized cost	126
Available for sale, with contractual maturities, amortized cost	528
Available for sale, due in one year or less, fair value	94
Available for sale, due after one year but within five years, fair value	236
Available for sale, due after five year but within ten years, fair value	68
Available for sale, due after ten years, fair value	125
Available for sale, with contractual maturities, fair value	523
Held to maturity, due in one year or less, amortized cost	42
Held to maturity, due after one year but within five years, amortized cost	185
Held to maturity, due after five year but within ten years, amortized cost	116
Held to maturity, due after ten years, amortized cost	955
Securities held to maturity, with contractual maturities, amortized cost	1,298
Held to maturity, due in one year or less, fair value	41
Held to maturity, due after one year but within five years, fair value	181
Held to maturity, due after five year but within ten years, fair value	116
Held to maturity, due after ten years, fair value	910
Securities held to maturity, with contractual maturities, fair value	1,248
Debt securities, available-for-sale, amortized cost	3,401
Debt Securities, Available-for-sale, Fair Value	3,341
Securities held to maturity, amortized cost	3,254
Securities held to maturity, Fair Value	3,155	$ 3,218
Agency mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, maturity, amortized cost	1,465
Debt securities, available-for-sale, fair value	1,429
Debt securities, available-for-sale, amortized cost	1,465	1,615
Debt Securities, Available-for-sale, Fair Value	1,429	1,599
Securities held to maturity, amortized cost	1,036
Securities held to maturity, Fair Value	1,010	1,214
Agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, maturity, amortized cost	1,179
Debt securities, available-for-sale, fair value	1,161
Debt securities, available-for-sale, amortized cost	1,179	813
Debt Securities, Available-for-sale, Fair Value	1,161	795
Securities held to maturity, amortized cost	794
Securities held to maturity, Fair Value	771	757
Commercial mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, maturity, amortized cost	229
Debt securities, available-for-sale, fair value	228
Debt securities, available-for-sale, amortized cost	229
Debt Securities, Available-for-sale, Fair Value	228
Securities held to maturity, amortized cost	126
Securities held to maturity, Fair Value	$ 126	$ 80