Securities - Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses, and Fair Values of Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities available for sale
Amortized Cost	$ 18,238	$ 14,378
Gross Unrealized Gains	77	54
Gross Unrealized Losses	201	291
Securities Available for Sale	18,114	14,141
U.S. Treasury
Securities available for sale
Amortized Cost	3,072	2,625
Gross Unrealized Gains	0	1
Gross Unrealized Losses	90	121
Securities Available for Sale	2,982	2,505
U.S. government-sponsored agencies
Securities available for sale
Amortized Cost	11
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Securities Available for Sale	12
U.S. government agency and government-sponsored agencies
Securities available for sale
Amortized Cost	10,117	6,814
Gross Unrealized Gains	11	3
Gross Unrealized Losses	82	122
Securities Available for Sale	10,046	6,695
Privately issued
Securities available for sale
Amortized Cost	552	333
Gross Unrealized Gains	3	1
Gross Unrealized Losses	4	7
Securities Available for Sale	551	327
Privately issued - commercial mortgage-backed securities
Securities available for sale
Amortized Cost	739	666
Gross Unrealized Gains	6	4
Gross Unrealized Losses	3	6
Securities Available for Sale	742	664
Collateralized loan obligations
Securities available for sale
Amortized Cost	2,133	2,219
Gross Unrealized Gains	9	4
Gross Unrealized Losses	1	5
Securities Available for Sale	2,141	2,218
Other
Securities available for sale
Amortized Cost	5	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities Available for Sale	5	7
Asset Liability Management securities
Securities available for sale
Amortized Cost	16,629	12,664
Gross Unrealized Gains	30	13
Gross Unrealized Losses	180	261
Securities Available for Sale	16,479	12,416
Direct bank purchase bonds
Securities available for sale
Amortized Cost	1,519	1,601
Gross Unrealized Gains	47	41
Gross Unrealized Losses	21	29
Securities Available for Sale	1,545	1,613
Other
Securities available for sale
Amortized Cost	80	108
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Securities Available for Sale	80	107
Equity securities
Securities available for sale
Amortized Cost	10	5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities Available for Sale	$ 10	$ 5

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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