Fair Value Measurements - Significant Unobservable Inputs Freestanding Derivatives (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Interest rate swaps | Long [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (28)	$ (29)
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%
Interest Rate Swaption [Member] | Long [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 1
Interest Rate Swaption [Member] | Long [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	3.00%
Interest Rate Swaption [Member] | Long [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	3.00%
Variance Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (25)	$ (26)
Variance Swap [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	21.00%	19.00%
Variance Swap [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	21.00%	19.00%
Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 38	$ 29
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	29.00%	31.00%
Other Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 56	$ 59
Other Contract [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	14.00%	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%
Hedge Funds, Equity [Member] | Equity index options
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net		$ 1
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%