Derivative Financial Instruments and Off-balance sheet Financial Instruments - Counterparty credit exposure and other (Details) $ in Millions	Dec. 31, 2018 USD ($) counter-party	Dec. 31, 2017 USD ($) counter-party
Credit Derivatives
Cash and securities pledged as collateral by counterparties	$ 8
Securities pledged as collateral to counterparties	2
Collateral posted under MNAs for contracts containing credit-risk-contingent features	$ 0	$ 3
Number of counter-parties | counter-party	4,000,000	2
Notional amount	$ 306	$ 59
Credit exposure	9	3
Exposure, net of collateral	1	0
Securities pledged in the form of margin deposits	7
Gross liability fair value of contracts containing credit-risk-contingent features	2	12
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(2)	(5)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	0	(3)
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	$ 0	$ 4
A plus
Credit Derivatives
Number of counter-parties | counter-party	3,000,000	2
Notional amount	$ 283	$ 59
Credit exposure	9	3
Exposure, net of collateral	$ 1	$ 0
A
Credit Derivatives
Number of counter-parties | counter-party	1,000,000	0
Notional amount	$ 23	$ 0
Credit exposure	0	0
Exposure, net of collateral	$ 0	$ 0