Assumption of Insured Portfolio and Business Combinations - Schedule of Effects of SGI Transaction (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
	Jun. 01, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Cash		$ 104				$ 144				$ 104	$ 144
Premiums receivable/payable, net of commissions		904				915				904	915
Unearned premium reserve, net		(3,512)				(3,475)				(3,512)	(3,475)
Credit derivative liability, net		(209)				(271)				(209)	(271)
Liabilities		7,048				7,594				7,048	7,594
Commutation loss		$ 0	$ (1)	$ 18	$ (1)	$ 0	$ (255)	$ 0	$ (73)	$ 16	$ (328)	$ (8)
Reinsurance of SGI Insured Portfolio [Member] | AGC [Member]
Business Acquisition [Line Items]
Cash	$ 363
Premiums receivable/payable, net of commissions	61
Unearned premium reserve, net	(375)
Credit derivative liability, net	(68)
Other	1
Liabilities	381
Commutation loss	18
Reinsurance of SGI Insured Portfolio [Member] | Commutation [Member] | AGC [Member]
Business Acquisition [Line Items]
Cash	20
Premiums receivable/payable, net of commissions	16
Unearned premium reserve, net	(56)
Credit derivative liability, net	0
Other	2
Liabilities	38
Commutation loss	18
Reinsurance of SGI Insured Portfolio [Member] | Insurance Contracts Assumed [Member] | AGC [Member]
Business Acquisition [Line Items]
Cash	343
Premiums receivable/payable, net of commissions	45
Unearned premium reserve, net	(319)
Credit derivative liability, net	(68)
Other	(1)
Liabilities	343
Commutation loss	$ 0