Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 627	$ 757	$ 700	$ 876
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(1)	4	3	32
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			0
Settlements	(30)	(36)	(90)	(117)
FG VIE consolidations				21
FG VIE deconsolidations		(18)	(17)	(105)
Transfers into Level 3				0
Fair value at end of period	596	707	596	707
Change in unrealized gains/(losses) related to financial instruments held	2	10	13	50
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	61	65	64	65
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(1)	(4)	(3)	(4)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			0
Settlements	0	0	(1)	0
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into Level 3				0
Fair value at end of period	60	61	60	61
Change in unrealized gains/(losses) related to financial instruments held	(1)	(4)	(3)	(4)
FG VIEs' liabilities with recourse, at fair value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	(571)	(689)	(627)	(807)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	3	(3)	(1)	(14)
Other comprehensive income (loss)	(3)	0	(1)	0
Purchases	0	0	0	0
Issuances			0
Settlements	26	35	83	113
FG VIE consolidations				0
FG VIE deconsolidations		0	1	51
Transfers into Level 3				0
Fair value at end of period	(545)	(657)	(545)	(657)
Change in unrealized gains/(losses) related to financial instruments held	(1)	(3)	(2)	(12)
Financial Guaranty Variable Interest Liabilities without Recourse [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	(108)	(131)	(130)	(151)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	1	(1)	3	(4)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			0
Settlements	3	3	7	11
FG VIE consolidations				(21)
FG VIE deconsolidations		18	16	54
Transfers into Level 3				0
Fair value at end of period	(104)	(111)	(104)	(111)
Change in unrealized gains/(losses) related to financial instruments held	1	(1)	2	(4)
Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	92	91	76	39
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	(8)	3	(6)
Other comprehensive income (loss)	6	(1)	17	(4)
Purchases	0	0	4	0
Issuances			0
Settlements	0	0	(1)	(2)
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into Level 3				55
Fair value at end of period	99	82	99	82
Change in unrealized gains/(losses) related to financial instruments held	6	(1)	17	(4)
Corporate securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	63	63	67	60
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	2	1	(2)	4
Other comprehensive income (loss)	(5)	2	(5)	2
Purchases	0	0	0	0
Issuances			0
Settlements	0	0	0	0
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into Level 3				0
Fair value at end of period	60	66	60	66
Change in unrealized gains/(losses) related to financial instruments held	(5)	2	(5)	2
RMBS [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	311	357	334	365
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	3	5	16	23
Other comprehensive income (loss)	0	(1)	(10)	25
Purchases	0	13	9	42
Issuances			0
Settlements	(15)	(28)	(50)	(109)
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into Level 3				0
Fair value at end of period	299	346	299	346
Change in unrealized gains/(losses) related to financial instruments held	1	0	(7)	25
Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	897	656	787	805
Acquisitions				7
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	14	15	43	100
Other comprehensive income (loss)	(12)	2	(3)	60
Purchases	64	106	164	162
Issuances			0
Settlements	(10)	(7)	(38)	(362)
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into Level 3				0
Fair value at end of period	953	772	953	772
Change in unrealized gains/(losses) related to financial instruments held	(11)	2	(1)	126
Credit Risk Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value at start of period	(257)	(361)	(269)	(389)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	21	58	103	106
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			(68)
Settlements	(1)	1	(3)	(19)
FG VIE consolidations				0
FG VIE deconsolidations		0	0	0
Transfers into level 3				0
Fair value at end of period	(237)	(302)	(237)	(302)
Change in unrealized gains/(losses) related to financial instruments held as of end of year	$ 20	$ 51	$ 93	$ 63