Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 526	$ 627	$ 569	$ 700
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	6	(1)	70	3
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances				0
Settlements	(69)	(30)	(170)	(90)
FG VIE consolidation	6		6
FG VIE deconsolidations			(6)	(17)
Transfers Into Level 3			0
Fair value at end of period	469	596	469	596
Change in unrealized gains/(losses) related to financial instruments held	6	2	78	13
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	87	61	77	64
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(14)	(1)	(4)	(3)
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances				0
Settlements	0	0	0	(1)
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers Into Level 3			0
Fair value at end of period	73	60	73	60
Change in unrealized gains/(losses) related to financial instruments held	(14)	(1)	(4)	(3)
Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	105	92	99	76
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	3	1	5	3
Other comprehensive income (loss)	(4)	6	1	17
Purchases	6	0	6	4
Issuances				0
Settlements	(1)	0	(2)	(1)
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers Into Level 3			0
Fair value at end of period	109	99	109	99
Change in unrealized gains/(losses) related to financial instruments held		6		17
Corporate securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	48	63	56	67
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	2	(9)	(2)
Other comprehensive income (loss)	(3)	(5)	(1)	(5)
Purchases	0	0	0	0
Issuances				0
Settlements	0	0	0	0
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers Into Level 3			0
Fair value at end of period	46	60	46	60
Change in unrealized gains/(losses) related to financial instruments held		(5)		(5)
RMBS [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	325	311	309	334
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	5	3	16	16
Other comprehensive income (loss)	1	0	21	(10)
Purchases	0	0	11	9
Issuances				0
Settlements	(15)	(15)	(41)	(50)
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers Into Level 3			0
Fair value at end of period	316	299	316	299
Change in unrealized gains/(losses) related to financial instruments held		1		(7)
Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	674	897	947	787
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	7	14	51	43
Other comprehensive income (loss)	(3)	(12)	(97)	(3)
Purchases	1	64	19	164
Issuances				0
Settlements	(15)	(10)	(257)	(38)
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers Into Level 3			1
Fair value at end of period	664	953	664	953
Change in unrealized gains/(losses) related to financial instruments held		(11)		(1)
Other Comprehensive Income (Loss) [Member] | Other Assets and Other Invested Assets [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	0		0
Other Comprehensive Income (Loss) [Member] | Obligations of state and political subdivisions [Member] | Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(4)		1
Other Comprehensive Income (Loss) [Member] | Corporate securities [Member] | Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(3)		(1)
Other Comprehensive Income (Loss) [Member] | RMBS [Member] | Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	1		21
Other Comprehensive Income (Loss) [Member] | Asset-backed Securities [Member] | Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(2)		8
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	(446)	(571)	(517)	(627)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(2)	3	(33)	(1)
Other comprehensive income (loss)	1	(3)	6	(1)
Purchases	0	0	0	0
Issuances				0
Settlements	64	26	156	83
FG VIE consolidation	(5)		(5)
FG VIE deconsolidations			5	1
Transfers Into Level 3			0
Fair value at end of period	(388)	(545)	(388)	(545)
Change in unrealized gains/(losses) included in earnings related to financial instruments	(2)	(1)	(32)	(2)
FG VIEs' liabilities with recourse, at fair value [Member] | Other Comprehensive Income (Loss) [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Change in unrealized gains/(losses) included in earnings related to financial instruments	1		6
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	(105)	(108)	(102)	(130)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(2)	1	(9)	3
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances				0
Settlements	3	3	6	7
FG VIE consolidation	(1)		(1)
FG VIE deconsolidations			1	16
Transfers Into Level 3			0
Fair value at end of period	(105)	(104)	(105)	(104)
Change in unrealized gains/(losses) included in earnings related to financial instruments	(1)	1	(16)	2
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	(216)	(257)	(207)	(269)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	5	21	(25)	103
Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Issuances				(68)
Settlements	3	(1)	24	(3)
FG VIE consolidation	0		0
FG VIE deconsolidations			0	0
Transfers into Level 3			0
Fair value at end of period	(208)	(237)	(208)	(237)
Change in unrealized gains/(losses) related to financial instruments held as of end of year	$ 8	$ 20	$ (19)	$ 93