Securities - Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses, and Fair Values of Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Securities available for sale
Amortized Cost	$ 17,621	$ 17,794
Gross Unrealized Gains	32	58
Gross Unrealized Losses	576	289
Securities Available for Sale	17,077	17,563
U.S. Treasury
Securities available for sale
Amortized Cost	3,964	3,370
Gross Unrealized Gains	0	0
Gross Unrealized Losses	234	118
Securities Available for Sale	3,730	3,252
U.S. government agency and government-sponsored agencies
Securities available for sale
Amortized Cost	8,739	9,338
Gross Unrealized Gains	2	2
Gross Unrealized Losses	240	132
Securities Available for Sale	8,501	9,208
Privately issued
Securities available for sale
Amortized Cost	891	695
Gross Unrealized Gains	0	3
Gross Unrealized Losses	30	4
Securities Available for Sale	861	694
Privately issued - commercial mortgage-backed securities
Securities available for sale
Amortized Cost	1,109	823
Gross Unrealized Gains	0	4
Gross Unrealized Losses	31	5
Securities Available for Sale	1,078	822
Collateralized loan obligations
Securities available for sale
Amortized Cost	1,434	1,895
Gross Unrealized Gains	1	10
Gross Unrealized Losses	1	0
Securities Available for Sale	1,434	1,905
Other
Securities available for sale
Amortized Cost	4	5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities Available for Sale	4	5
Asset Liability Management securities
Securities available for sale
Amortized Cost	16,141	16,126
Gross Unrealized Gains	3	19
Gross Unrealized Losses	536	259
Securities Available for Sale	15,608	15,886
Direct bank purchase bonds
Securities available for sale
Amortized Cost	1,292	1,495
Gross Unrealized Gains	28	38
Gross Unrealized Losses	39	30
Securities Available for Sale	1,281	1,503
Other
Securities available for sale
Amortized Cost	188	163
Gross Unrealized Gains	1	1
Gross Unrealized Losses	1	0
Securities Available for Sale	$ 188	164
Equity securities
Securities available for sale
Amortized Cost		10
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Securities Available for Sale		$ 10

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=75030427&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
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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.

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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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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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