Derivative Instruments - Derivatives - Credit Risk Assumed through Credit Derivatives (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Notional Amount	$ 12,629	$ 12,730
Fair Value	(60)	(90)
Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Notional Amount	999	1,346
Credit Risk Derivatives, at Fair Value, Net	4	0
Credit derivatives
Credit Derivatives [Line Items]
Offsetting Notional Amount	26	523
Credit derivatives | Single name credit default swaps | A Minus | Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	169	130
Fair Value	$ 3	$ 3
Weighted Average Years to Maturity	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Credit derivatives | Single name credit default swaps | B | Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		9
Fair Value		0
Offsetting Notional Amount		9
Offsetting Fair Value		0
Credit derivatives | Basket credit default swaps | A Minus | Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	12
Fair Value	$ 0
Weighted Average Years to Maturity	5 years
Offsetting Notional Amount	$ 2
Offsetting Fair Value	0
Credit derivatives | Basket credit default swaps | BBB Plus | Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	794	1,137
Fair Value	$ 7	$ 2
Weighted Average Years to Maturity	6 years	3 years
Offsetting Notional Amount	$ 0	$ 454
Offsetting Fair Value	0	(2)
Credit derivatives | Basket credit default swaps | B Plus | Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		27
Fair Value		$ 2
Weighted Average Years to Maturity		3 years
Offsetting Notional Amount		$ 27
Offsetting Fair Value		0
Credit derivatives | Basket credit default swaps | A | Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		13
Fair Value		$ (1)
Weighted Average Years to Maturity		5 years
Offsetting Notional Amount		$ 3
Offsetting Fair Value		0
Credit derivatives | Basket credit default swaps | CCC | Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	24	30
Fair Value	(6)	(6)
Offsetting Notional Amount	24	30
Offsetting Fair Value	6	7
Credit derivatives in offsetting positions | Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Credit Risk Derivatives, at Fair Value, Net	$ 6	$ 5