Changes in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
AOCI [Line Items]
AOCI	$ (1,028)	$ (751)	$ (593)	$ (559)
OCI before reclassifications, net of tax	(416)	(239)	(208)
Personnel expense	3,469	3,180	3,293
Interest income	6,327	6,142	6,507
Interest expense	735	768	891
FDIC loss share income, net	(253)	(343)	(293)
Securities (gains) losses, net	3	3	(51)
Income before income taxes	2,917	3,127	3,283
Income taxes	794	921	1,553
Net income	2,123	2,206	1,730
Net change in AOCI	(277)	(158)	(34)
Amounts reclassified from AOCI [Member]
AOCI [Line Items]
Personnel expense	67	17	91
Interest income	38	(18)	99
Interest expense	83	82	77
FDIC loss share income, net	31	45	63
Securities (gains) losses, net	3	3	(51)
Income before income taxes	222	129	279
Income taxes	83	48	105
Net income	139	81	174
Unrecognized Net Pension and Postretirement Costs
AOCI [Line Items]
AOCI	(723)	(626)	(303)	(714)
OCI before reclassifications, net of tax	(139)	(334)	354
Net change in AOCI	(97)	(323)	411
Unrecognized Net Pension and Postretirement Costs | Amounts reclassified from AOCI [Member]
AOCI [Line Items]
Personnel expense	67	17	91
Income before income taxes	67	17	91
Income taxes	25	6	34
Net income	42	11	57
Unrealized Net Gain (Loss) on Cash Flow Hedges
AOCI [Line Items]
AOCI	(83)	(54)	2	(173)
OCI before reclassifications, net of tax	(81)	(107)	127
Net change in AOCI	(29)	(56)	175
Unrealized Net Gain (Loss) on Cash Flow Hedges | Amounts reclassified from AOCI [Member]
AOCI [Line Items]
Interest expense	83	82	77
Income before income taxes	83	82	77
Income taxes	31	31	29
Net income	52	51	48
Unrealized Net Gains (Losses) on AFS Securities [Member]
AOCI [Line Items]
AOCI	(34)	152	(42)	598
OCI before reclassifications, net of tax	(206)	207	(669)
Net change in AOCI	(186)	194	(640)
Unrealized Net Gains (Losses) on AFS Securities [Member] | Amounts reclassified from AOCI [Member]
AOCI [Line Items]
Interest income	29	(24)	97
Securities (gains) losses, net	3	3	(51)
Income before income taxes	32	(21)	46
Income taxes	12	(8)	17
Net income	20	(13)	29
FDIC's Share Of Unrealized (Gains) Losses On AFS Securities
AOCI [Line Items]
AOCI	(169)	(207)	(235)	(256)
OCI before reclassifications, net of tax	19	0	(18)
Net change in AOCI	38	28	21
FDIC's Share Of Unrealized (Gains) Losses On AFS Securities | Amounts reclassified from AOCI [Member]
AOCI [Line Items]
FDIC loss share income, net	31	45	63
Income before income taxes	31	45	63
Income taxes	12	17	24
Net income	19	28	39
Other, Net [Member]
AOCI [Line Items]
AOCI	(19)	(16)	(15)	$ (14)
OCI before reclassifications, net of tax	(9)	(5)	(2)
Net change in AOCI	(3)	(1)	(1)
Other, Net [Member] | Amounts reclassified from AOCI [Member]
AOCI [Line Items]
Interest income	9	6	2
Income before income taxes	9	6	2
Income taxes	3	2	1
Net income	$ 6	$ 4	$ 1