Fair Value Measurement and Fair Value of Financial Instruments - Significant Unobservable Inputs Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Unobservable Input(s) Level 3
Securities Available for Sale	$ 17,077	$ 17,563
Direct bank purchase bonds
Unobservable Input(s) Level 3
Securities Available for Sale	$ 1,281	$ 1,503
Direct bank purchase bonds \| Weighted Average \| Return on equity
Unobservable Input(s) Level 3
Market-required return on capital (as a percent)	9.50%
Probability of default (as a percent)	0.30%
Loss severity (as a percent)	24.30%
Direct bank purchase bonds \| Level 3
Unobservable Input(s) Level 3
Securities Available for Sale	$ 1,281

[Line Items] for Fair Value Inputs, Assets and Liabilities Quantitative Information

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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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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

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Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

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