Short-Term Borrowings - Components of Short-Term Borrowings (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Balance at year end, Securities Sold Under Repurchase Agreements	$ 0	$ 0
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Balance at year end, Federal Funds Purchase	1,005	20	$ 18
Average during the year	44	195	32
Maximum month-end balance	$ 1,005	$ 678	$ 36
Weighted-average rate during the year	0.68%	0.14%	0.10%
Weighted-average rate at December 31	0.55%	0.05%	0.08%
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Average during the year	$ 443	$ 437	$ 1,150
Maximum month-end balance	$ 684	$ 589	$ 1,519
Weighted-average rate during the year	0.04%	0.00%	0.16%
Weighted-average rate at December 31	0.07%	0.00%	0.01%
Balance at year end, Securities Sold Under Repurchase Agreements	$ 497	$ 352	$ 557
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Average during the year	852	572	597
Maximum month-end balance	$ 872	$ 1,122	$ 996
Weighted-average rate during the year	1.18%	1.52%	1.49%
Weighted-average rate at December 31	1.11%	1.78%	1.58%
Balance at year end, Other Short-Term Borrowings	$ 808	$ 533	$ 423