Fair Value Measurements - Significant Unobservable Inputs Freestanding Derivatives (Details) - Fair Value, Inputs, Level 3 [Member] $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Hedge Funds, Equity [Member] | Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net	$ (2)
Fair Value Measurements, Sensitivity Analysis, Description	Increase
Derivative Liability, Measurement Input	Option model
Hedge Funds, Equity [Member] | Equity index options | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.30
Hedge Funds, Equity [Member] | Equity index options | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.32
Hedge Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net		$ 29
Fair Value Measurements, Sensitivity Analysis, Description		Increase
Hedge Funds [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Liability, Measurement Input		Option model
Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net	$ (36)	$ 1
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Equity index options | Valuation Technique, Option Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Liability, Measurement Input	Option model	Option model
Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net	$ 32	$ 59
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Swap [Member] | Valuation Technique, Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Liability, Measurement Input	Discounted cash flows	Discounted cash flows
Variance Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net	$ (27)	$ (26)
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Variance Swap [Member] | Valuation Technique, Option Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Liability, Measurement Input	Option model	Option model
Interest rate swaps
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Assets (Liabilities), at Fair Value, Net	$ (26)	$ (29)
Fair Value Measurements, Sensitivity Analysis, Description	Decrease	Decrease
Interest rate swaps | Valuation Technique, Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Liability, Measurement Input	Discounted cash flows	Discounted cash flows
Measurement Input, Discount Rate [Member] | Interest rate swaps | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.03	0.02
Measurement Input, Discount Rate [Member] | Interest rate swaps | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.03	0.03
Measurement Input, Option Volatility [Member] | Hedge Funds [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input		0.18
Measurement Input, Option Volatility [Member] | Hedge Funds [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input		0.31
Measurement Input, Option Volatility [Member] | Equity index options | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.18	0.27
Measurement Input, Option Volatility [Member] | Equity index options | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.29	0.30
Measurement Input, Option Volatility [Member] | Swap [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.08	0.07
Measurement Input, Option Volatility [Member] | Swap [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.30	0.30
Measurement Input, Option Volatility [Member] | Variance Swap [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.18	0.19
Measurement Input, Option Volatility [Member] | Variance Swap [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Derivative Asset (Liability) Net, Measurement Input	0.18	0.19