Available-for-Sale Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	$ 20,847	$ 14,253
Available-for-sale securities, gross unrealized gains	96	42
Available-for-sale securities, gross unrealized losses	(264)	(403)
Available-for-sale securities, fair value	20,679	13,892
Transfer of available-for-sale securities to held-to-maturity	0	492	$ 0
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	20,840	14,246
Available-for-sale securities, gross unrealized gains	96	42
Available-for-sale securities, gross unrealized losses	(264)	(403)
Available-for-sale securities, fair value	20,672	13,885
Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	19,395	12,946
Available-for-sale securities, gross unrealized gains	47	24
Available-for-sale securities, gross unrealized losses	(247)	(336)
Available-for-sale securities, fair value	19,195	12,634
Agency Debentures [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	939	791
Available-for-sale securities, gross unrealized gains	39	18
Available-for-sale securities, gross unrealized losses	(12)	(21)
Available-for-sale securities, fair value	966	788
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	452	452
Available-for-sale securities, gross unrealized gains	10	0
Available-for-sale securities, gross unrealized losses	(4)	(45)
Available-for-sale securities, fair value	458	407
Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	34	25
Available-for-sale securities, gross unrealized gains	0	0
Available-for-sale securities, gross unrealized losses	(1)	(1)
Available-for-sale securities, fair value	33	24
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	20	32
Available-for-sale securities, gross unrealized gains	0	0
Available-for-sale securities, gross unrealized losses	0	0
Available-for-sale securities, fair value	20	32
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	7	7
Available-for-sale securities, gross unrealized losses	0	0
Available-for-sale securities, fair value	$ 7	$ 7

Fair value of available-for-sale securities transferred to held-to-maturity during the period.

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- Definition

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

+ References

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
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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.

+ References

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