Business Combinations and Assumption of Insured Portfolio - Schedule of Effects of SGI Transaction (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
	Jun. 01, 2018	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
Business Acquisition [Line Items]
Cash		$ 169				$ 104				$ 169	$ 104	$ 144
Premiums receivable/payable, net of commissions		1,286				904				1,286	904
Unearned premium reserve, net		(3,736)				(3,512)				(3,736)	(3,512)
Credit derivative liability, net		(191)				(209)				(191)	(209)
Other		511				298				511	298
Impact to net assets (liabilities), excluding cash		(7,674)				(7,048)				(7,674)	(7,048)
Commutation loss		$ 0	$ 0	$ (1)	$ 0	$ 0	$ (1)	$ 18	$ (1)	$ (1)	$ 16	$ (328)
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
Impact to net assets (liabilities), excluding cash	(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
Impact to net assets (liabilities), excluding cash	(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)
Impact to net assets (liabilities), excluding cash	(343)
Commutation loss	$ 0