Assumption of Insured Portfolio and Business Combinations - Schedule of Effects of SGI Transaction (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jun. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Cash		$ 82	$ 72	$ 82	$ 72	$ 144	$ 118
Premiums receivable, net of commissions payable		916		916		915
Unearned premium reserve		(3,538)		(3,538)		(3,475)
Credit derivative liability, net		(239)		(239)		(271)
Total liabilities		(7,156)		(7,156)		$ (7,594)
Commutation loss		$ (1)	$ (255)	$ 16	$ (328)
AGC [Member] | Reinsurance of SGI Insured Portfolio [Member]
Business Acquisition [Line Items]
Cash	$ 363
Premiums receivable, net of commissions payable	61
Unearned premium reserve	(375)
Credit derivative liability, net	(68)
Other	1
Total liabilities	(381)
Commutation loss	18
Commutation [Member] | AGC [Member] | Reinsurance of SGI Insured Portfolio [Member]
Business Acquisition [Line Items]
Cash	20
Premiums receivable, net of commissions payable	16
Unearned premium reserve	(56)
Credit derivative liability, net	0
Other	2
Total liabilities	(38)
Commutation loss	18
Assumption [Member] | AGC [Member] | Reinsurance of SGI Insured Portfolio [Member]
Business Acquisition [Line Items]
Cash	343
Premiums receivable, net of commissions payable	45
Unearned premium reserve	(319)
Credit derivative liability, net	(68)
Other	(1)
Total liabilities	(343)
Commutation loss	$ 0