Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Short-term Investments [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period				$ 60
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 deconsolidations				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	$ 781	1,191	$ 876	1,261
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	11	113	28	109
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	(35)	(490)	(81)	(556)
FG VIE Consolidations			21
FG VIE deconsolidations			(87)	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	757	814	757	814
Change in unrealized gains/(losses) related to financial instruments held	19	11	40	15
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	63	49	65	65
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	2	(11)	0	(27)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	0	0
FG VIE Consolidations			0
FG VIE deconsolidations			0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	65	38	65	38
Change in unrealized gains/(losses) related to financial instruments held	2	(11)	0	(27)
Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	42	7	39	8
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	0	2	0
Other comprehensive income (loss)	(7)	1	(3)	1
Purchases	0	33	0	33
Settlements	0	0	(2)	(1)
FG VIE Consolidations			0
FG VIE deconsolidations			0	0
Transfers into Level 3	55		55
Transfers out of Level 3	0
Fair value at end of period	91	41	91	41
Change in unrealized gains/(losses) related to financial instruments held	9	1	13	1
Corporate securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	62	74	60	71
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	2	3	4
Other comprehensive income (loss)	0	(18)	0	(17)
Purchases	0	0	0	0
Settlements	0	0	0	0
FG VIE Consolidations			0
FG VIE deconsolidations			0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	63	58	63	58
Change in unrealized gains/(losses) related to financial instruments held	0	(18)	0	(17)
RMBS [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	402	360	365	348
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	20	4	18	2
Other comprehensive income (loss)	(1)	3	26	(2)
Purchases	2	6	29	40
Settlements	(66)	(24)	(81)	(39)
FG VIE Consolidations			0
FG VIE deconsolidations			0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	357	349	357	349
Change in unrealized gains/(losses) related to financial instruments held	13	2	26	(3)
Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	602	639	805	657
Acquisitions			7
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	11	10	85	11
Other comprehensive income (loss)	51	(5)	58	(10)
Purchases	4	0	56	0
Settlements	(7)	(80)	(355)	(94)
FG VIE Consolidations			0
FG VIE deconsolidations			0	0
Transfers into Level 3	0		0
Transfers out of Level 3	(5)
Fair value at end of period	656	564	656	564
Change in unrealized gains/(losses) related to financial instruments held	51	(4)	124	(10)
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	(721)	(1,165)	(807)	(1,225)
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(2)	(112)	(11)	(91)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	34	487	78	526
FG VIE consolidations			0
FG VIE deconsolidations			51	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	(689)	(790)	(689)	(790)
Change in unrealized gains/(losses) related to financial instruments held	(2)	(2)	(9)	19
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	(134)	(119)	(151)	(124)
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(1)	2	(3)	4
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	4	2	8	5
FG VIE consolidations			(21)
FG VIE deconsolidations			36	0
Transfers into Level 3	0		0
Transfers out of Level 3	0
Fair value at end of period	(131)	(115)	(131)	(115)
Change in unrealized gains/(losses) related to financial instruments held	(1)	2	(3)	3
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	(350)	(434)	(389)	(365)
Acquisitions			0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	(6)	63	48	3
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Settlements	(5)	(25)	(20)	(34)
FG VIE consolidations			0
PG VIE deconsolidations			0	0
Transfers into Level 3	0		0
Transfers out of level 3	0
Fair value at end of period	(361)	(396)	(361)	(396)
Change in unrealized gains/(losses) related to financial instruments held as of June 30, 2017	$ (13)	$ (20)	$ 12	$ (99)