Assumption of Insured Portfolio and Business Combinations - Schedule of Effects of SGI Transaction (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jun. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Cash		$ 185	$ 200	$ 185	$ 200	$ 144	$ 118
Premiums receivable, net of commissions payable		932		932		915
Unearned premium reserve		(3,635)		(3,635)		(3,475)
Credit derivative liabilities		(258)		(258)		(271)
Deferred acquisition costs		102		102		101
Total liabilities		7,498		7,498		$ 7,594
Commutation gains (losses)		$ (18)	$ 0	$ (17)	$ 73
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 liabilities	(68)
Deferred acquisition costs	1
Total liabilities	381
Commutation gains (losses)	(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 liabilities	0
Deferred acquisition costs	2
Total liabilities	38
Commutation gains (losses)	(18)
Assumed [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 liabilities	(68)
Deferred acquisition costs	(1)
Total liabilities	343
Commutation gains (losses)	$ 0