Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Short-term Investments [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period				$ 60
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)				0
Other comprehensive income (loss)				0
Purchases				0
Settlements				(60)
FG VIE Consolidations				0
FG VIE deconsolidations				0
Transfers into Level 3				0
Fair value at end of period		$ 0		0
Change in unrealized gains/(losses) related to financial instruments held				0
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 757	814	$ 876	1,261
Acquisitions		0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	4	20	32	129
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	(36)	(34)	(117)	(590)
FG VIE Consolidations		97	21	97
FG VIE deconsolidations	(18)	(20)	(105)	(20)
Transfers into Level 3	0	0	0	0
Fair value at end of period	707	877	707	877
Change in unrealized gains/(losses) related to financial instruments held	10	29	50	44
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	65	38	65	65
Acquisitions		0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(4)	(23)	(4)	(50)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	0	0
FG VIE Consolidations		0	0	0
FG VIE deconsolidations	0	0	0	0
Transfers into Level 3	0	0	0	0
Fair value at end of period	61	15	61	15
Change in unrealized gains/(losses) related to financial instruments held	(4)	(23)	(4)	(50)
Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	91	41	39	8
Acquisitions		1	0	1
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(8)	1	(6)	1
Other comprehensive income (loss)	(1)	0	(4)	1
Purchases	0	0	0	33
Settlements	0	0	(2)	(1)
FG VIE Consolidations		0	0	0
FG VIE deconsolidations	0	0	0	0
Transfers into Level 3	0	0	55	0
Fair value at end of period	82	43	82	43
Change in unrealized gains/(losses) related to financial instruments held	(1)	0	(4)	1
Corporate securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	63	58	60	71
Acquisitions		0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	0	4	4
Other comprehensive income (loss)	2	0	2	(17)
Purchases	0	0	0	0
Settlements	0	0	0	0
FG VIE Consolidations		0	0	0
FG VIE deconsolidations	0	0	0	0
Transfers into Level 3	0	0	0	0
Fair value at end of period	66	58	66	58
Change in unrealized gains/(losses) related to financial instruments held	2	0	2	(17)
RMBS [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	357	349	365	348
Acquisitions		20	0	20
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	5	3	23	5
Other comprehensive income (loss)	(1)	2	25	0
Purchases	13	24	42	64
Settlements	(28)	(15)	(109)	(54)
FG VIE Consolidations		0	0	0
FG VIE deconsolidations	0	0	0	0
Transfers into Level 3	0	0	0	0
Fair value at end of period	346	383	346	383
Change in unrealized gains/(losses) related to financial instruments held	0	1	25	(1)
Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	656	564	805	657
Acquisitions		36	7	36
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	15	9	100	20
Other comprehensive income (loss)	2	47	60	37
Purchases	106	53	162	53
Settlements	(7)	(40)	(362)	(134)
FG VIE Consolidations		0	0	0
FG VIE deconsolidations	0	0	0	0
Transfers into Level 3	0	22	0	22
Fair value at end of period	772	691	772	691
Change in unrealized gains/(losses) related to financial instruments held	2	47	126	37
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	(689)	(790)	(807)	(1,225)
Acquisitions		0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(3)	(21)	(14)	(112)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	35	33	113	559
FG VIE consolidations		(54)	0	(54)
FG VIE deconsolidations	0	0	51	0
Transfers into Level 3	0	0	0	0
Fair value at end of period	(657)	(832)	(657)	(832)
Change in unrealized gains/(losses) related to financial instruments held	(3)	(18)	(12)	1
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	(131)	(115)	(151)	(124)
Acquisitions		0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(1)	(18)	(4)	(14)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	3	3	11	8
FG VIE consolidations		(43)	(21)	(43)
FG VIE deconsolidations	18	20	54	20
Transfers into Level 3	0	0	0	0
Fair value at end of period	(111)	(153)	(111)	(153)
Change in unrealized gains/(losses) related to financial instruments held	(1)	(17)	(4)	(14)
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	(361)	(396)	(389)	(365)
Acquisitions		(67)	0	(67)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	58	21	106	24
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	1	(39)	(19)	(73)
Transfers into Level 3	0	0	0	0
FG VIE consolidations		0	0	0
PG VIE deconsolidations	0	0	0	0
Fair value at end of period	(302)	(481)	(302)	(481)
Change in unrealized gains/(losses) related to financial instruments held as of June 30, 2017	$ 51	$ (5)	$ 63	$ (104)