Contracts Accounted for as Credit Derivatives - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) Counterparty	Dec. 31, 2017 USD ($)	Jun. 01, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Credit Derivatives
Net Par Outstanding	$ 6,681	$ 6,207
Credit risk derivatives, at fair value, net	(257)	(269)
Net expected loss to be paid after recoveries	$ 1,432	$ 1,303		$ 1,298	$ 1,297	$ 1,244	$ 1,198
Estimated remaining weighted average life of credit derivatives (in years)	11 years 9 months 6 days	11 years 8 months 4 days
Gross par of CDS with collateral posting requirement [Member]
Credit Derivatives
Number of counterparties | Counterparty	1
Reinsurance of SGI Insured Portfolio [Member] | United States [Member] | SGI Portfolio [Member]
Credit Derivatives
Net Par Outstanding			$ 1,500
Credit risk derivatives, at fair value, net			(68)
Net expected loss to be paid after recoveries			$ 0