Securities - Gross unrealized losses and related fair values (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014 security	Dec. 31, 2013 security
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	63	262
Securities available-for-sale gross unrealized losses one year or less	$ (112)	$ (618)
Securities available-for-sale aggregate fair value of investment one year or less	5,889	28,250
Number of Securities available for sale greater than one year	81	53
Securities available-for-sale gross unrealized losses greater than one year	(54)	(109)
Securities available-for-sale aggregate fair value of investment greater than one year	4,873	4,227
Number of securities held-to-maturity in one year or less	329	99
Securities held-to-maturity gross unrealized losses one year or less	(1)	0
Securities held-to-maturity aggregate fair value of investment one year or less	2,466	4
Securities held-to-maturity greater than one year	861	909
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	3	3
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	10	22
Securities available-for-sale gross unrealized losses one year or less	(14)	(82)
Securities available-for-sale aggregate fair value of investment one year or less	3,515	16,958
Number of Securities available for sale greater than one year	5	6
Securities available-for-sale gross unrealized losses greater than one year	(20)	(31)
Securities available-for-sale aggregate fair value of investment greater than one year	1,582	630
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	13	23
Securities available-for-sale gross unrealized losses one year or less	(2)	(12)
Securities available-for-sale aggregate fair value of investment one year or less	361	400
Number of Securities available for sale greater than one year	25	20
Securities available-for-sale gross unrealized losses greater than one year	(11)	(19)
Securities available-for-sale aggregate fair value of investment greater than one year	432	356
Number of securities held-to-maturity in one year or less	213	13
Securities held-to-maturity gross unrealized losses one year or less	(1)	0
Securities held-to-maturity aggregate fair value of investment one year or less	1,200	0
Securities held-to-maturity greater than one year	49	48
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	0	0
US Government agency issued or guaranteed [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	6	170
Securities available-for-sale gross unrealized losses one year or less	(80)	(494)
Securities available-for-sale aggregate fair value of investment one year or less	891	10,243
Number of Securities available for sale greater than one year	10	5
Securities available-for-sale gross unrealized losses greater than one year	(3)	(2)
Securities available-for-sale aggregate fair value of investment greater than one year	62	23
Number of securities held-to-maturity in one year or less	116	79
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	1,266	0
Securities held-to-maturity greater than one year	809	859
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	2	2
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	23	42
Securities available-for-sale gross unrealized losses one year or less	(2)	(19)
Securities available-for-sale aggregate fair value of investment one year or less	141	330
Number of Securities available for sale greater than one year	29	5
Securities available-for-sale gross unrealized losses greater than one year	(5)	(6)
Securities available-for-sale aggregate fair value of investment greater than one year	285	65
Number of securities held-to-maturity in one year or less	0	7
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	0	4
Securities held-to-maturity greater than one year	3	2
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	1	1
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	5	3
Securities available-for-sale gross unrealized losses one year or less	(11)	(6)
Securities available-for-sale aggregate fair value of investment one year or less	129	115
Number of Securities available for sale greater than one year	6	10
Securities available-for-sale gross unrealized losses greater than one year	(8)	(36)
Securities available-for-sale aggregate fair value of investment greater than one year	93	237
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	5	1
Securities available-for-sale gross unrealized losses one year or less	0	0
Securities available-for-sale aggregate fair value of investment one year or less	696	50
Number of Securities available for sale greater than one year	6	7
Securities available-for-sale gross unrealized losses greater than one year	(7)	(15)
Securities available-for-sale aggregate fair value of investment greater than one year	2,419	2,916
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	1	1
Securities available-for-sale gross unrealized losses one year or less	(3)	(5)
Securities available-for-sale aggregate fair value of investment one year or less	156	154
Number of Securities available for sale greater than one year	0	0
Securities available-for-sale gross unrealized losses greater than one year	0	0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 0	$ 0