Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Issuances	$ (68)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Issuances	0
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	651	$ 781	$ 700	$ 876
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	3	11	4	28
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances	0		0
Settlements	(27)	(35)	(60)	(81)
FG VIE consolidations				(21)
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE deconsolidations			(17)	(87)
Fair value at end of period	627	757	627	757
Change in unrealized gains/(losses) related to financial instruments held	6	19	10	40
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				(21)
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	62	63	64	65
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(1)	2	(2)	0
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances	0		0
Settlements	0	0	(1)	0
FG VIE consolidations				0
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE deconsolidations			0	0
Fair value at end of period	61	65	61	65
Change in unrealized gains/(losses) related to financial instruments held	(1)	2	(2)	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				0
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	(598)	(721)	(627)	(807)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(4)	(2)	(4)	(11)
Other comprehensive income (loss)	4	0	2	0
Purchases	0	0	0	0
Issuances			0
Settlements	27	34	57	78
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE consolidations				0
FG VIE deconsolidations			1	51
Fair value at end of period	(571)	(689)	(571)	(689)
Change in unrealized gains/(losses) related to financial instruments held	2	(2)	(1)	(9)
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	(110)	(134)	(130)	(151)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	1	(1)	2	(3)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			0
Settlements	1	4	4	8
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE consolidations				(21)
FG VIE deconsolidations			16	36
Fair value at end of period	(108)	(131)	(108)	(131)
Change in unrealized gains/(losses) related to financial instruments held	1	(1)	1	(3)
Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	83	42	76	39
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	1	2	2
Other comprehensive income (loss)	8	(7)	11	(3)
Purchases	0	0	4	0
Issuances	0		0
Settlements	0	0	(1)	(2)
FG VIE consolidations				0
Transfers into level 3		55		55
Transfers out of Level 3		0
FG VIE deconsolidations			0	0
Fair value at end of period	92	91	92	91
Change in unrealized gains/(losses) related to financial instruments held	8	9	11	13
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				0
Corporate securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	62	62	67	60
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	1	(4)	3
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances	0		0
Settlements	0	0	0	0
FG VIE consolidations				0
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE deconsolidations			0	0
Fair value at end of period	63	63	63	63
Change in unrealized gains/(losses) related to financial instruments held	0	0	0	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				0
RMBS [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	314	402	334	365
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	6	20	13	18
Other comprehensive income (loss)	(3)	(1)	(10)	26
Purchases	9	2	9	29
Issuances	0		0
Settlements	(15)	(66)	(35)	(81)
FG VIE consolidations				0
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE deconsolidations			0	0
Fair value at end of period	311	357	311	357
Change in unrealized gains/(losses) related to financial instruments held	(2)	13	(8)	26
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				0
Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	809	602	787	805
Acquisitions				7
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	14	11	29	85
Other comprehensive income (loss)	6	51	9	58
Purchases	91	4	100	56
Issuances	0		0
Settlements	(23)	(7)	(28)	(355)
FG VIE consolidations				0
Transfers into level 3		0		0
Transfers out of Level 3		(5)
FG VIE deconsolidations			0	0
Fair value at end of period	897	656	897	656
Change in unrealized gains/(losses) related to financial instruments held	6	51	10	124
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
FG VIE consolidations				0
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	(236)	(350)	(269)	(389)
Acquisitions				0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	48	(6)	82	48
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances			(68)
Settlements	(1)	(5)	(2)	(20)
Transfers into level 3		0		0
Transfers out of Level 3		0
FG VIE consolidations				0
PG VIE deconsolidations			0	0
Fair value at end of period	(257)	(361)	(257)	(361)
Change in unrealized gains/(losses) related to financial instruments held as of June 30, 2017	$ 46	$ (13)	$ 73	$ 12