Fair Value Measurements - Significant Unobservable Inputs Freestanding Derivatives (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Interest rate swaps \| Long [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (27)	$ (29)
Fair Value Measurements, Valuation Techniques	Discounted cash flows	Discounted cash flows
Fair Value Measurements, Significant Assumptions	Swap curve beyond 30 years	Swap curve beyond 30 years
Fair Value Measurements, Sensitivity Analysis, Description	Decrease	Decrease
Interest rate swaps \| Long [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	3.00%	2.00%
Interest rate swaps \| Long [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	3.00%	3.00%
Variance Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (26)	$ (26)
Fair Value Measurements, Valuation Techniques	Option model	Option model
Fair Value Measurements, Significant Assumptions	Equity volatility	Equity volatility
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Variance Swap [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	19.00%	19.00%
Variance Swap [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	19.00%	19.00%
Other Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (1)	$ 59
Fair Value Measurements, Valuation Techniques	Discounted cash flows	Discounted cash flows
Fair Value Measurements, Significant Assumptions	Equity volatility	Equity volatility
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Other Contract [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	12.00%	7.00%
Other Contract [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	30.00%	30.00%
Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (1)	$ 29
Fair Value Measurements, Valuation Techniques	Option model	Option model
Fair Value Measurements, Significant Assumptions	Equity volatility	Equity volatility
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Equity index options \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	19.00%	18.00%
Equity index options \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	28.00%	31.00%
Interest Rate Swaption [Member] \| Long [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 44
Fair Value Measurements, Valuation Techniques	Option model
Fair Value Measurements, Significant Assumptions	Interest rate volatility
Fair Value Measurements, Sensitivity Analysis, Description	Increase
Interest Rate Swaption [Member] \| Long [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	28.00%
Interest Rate Swaption [Member] \| Long [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	29.00%
Hedge Funds, Equity [Member] \| Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net		$ 1
Fair Value Measurements, Valuation Techniques		Option model
Fair Value Measurements, Significant Assumptions		Equity volatility
Fair Value Measurements, Sensitivity Analysis, Description		Increase
Hedge Funds, Equity [Member] \| Equity index options \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate		27.00%
Hedge Funds, Equity [Member] \| Equity index options \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate		30.00%

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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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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No definition available.

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Description of the sensitivity of the fair value measurements to changes in unobservable inputs and the inter-relationships between those inputs as well as how those inter-relationships might magnify or mitigate the effect of changes in the unobservable inputs on the fair value measurements.

+ 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 (g)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

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