Fair Value Measurements - Significant Unobservable Inputs Freestanding Derivatives (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Interest Rate Swaption \| Long
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 1	$ 9
Interest Rate Swaption \| Minimum \| Long
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Interest rate volatility	2.00%	2.00%
Interest Rate Swaption \| Maximum \| Long
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Interest rate volatility	2.00%	2.00%
Equity options
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 1
Equity options \| Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Interest rate volatility	18.00%
Equity options \| Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Interest rate volatility	22.00%

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ 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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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

No definition available.

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