Segment Information - Reconciliation of Total GAAP Revenues to Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total GAAP revenues									$ 963	$ 1,001	$ 1,739
Less: Realized gains (losses) on investments	$ (10)	$ (16)	$ (8)	$ 12	$ 18	$ 7	$ 2	$ 5	(22)	32	(40)
Less: Non-credit impairment unrealized fair value gains (losses) on credit derivatives									21	128	121
Less: Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves									21	(35)	64
Plus: Credit derivative losses									35	25	27
Total segment revenues									964	959	1,558
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total GAAP revenues									963	1,001	1,739
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Less: Realized gains (losses) on investments									(22)	32	(40)
Less: Non-credit impairment unrealized fair value gains (losses) on credit derivatives									(10)	101	43
Less: Fair value gains (losses) on CCS									22	(14)	2
Less: Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves									22	(32)	57
Plus: Credit derivative losses									13	9	(43)
Total segment revenues									$ 22	$ (5)	$ 7