Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 700	$ 876
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	17
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	(33)	(46)
FG VIE Consolidations		21
FG VIE deconsolidations	(17)	(87)
Fair value at end of period	651	781
Change in unrealized gains/(losses) related to financial instruments held	4	21
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	64	65
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(1)	(2)
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	(1)	0
FG VIE Consolidations		0
FG VIE deconsolidations	0	0
Fair value at end of period	62	63
Change in unrealized gains/(losses) related to financial instruments held	(1)	(2)
Obligations of state and political subdivisions [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	76	39
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	1
Other comprehensive income (loss)	3	4
Purchases	4	0
Settlements	(1)	(2)
FG VIE Consolidations		0
FG VIE deconsolidations	0	0
Fair value at end of period	83	42
Change in unrealized gains/(losses) related to financial instruments held	3	4
Corporate securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	67	60
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(5)	2
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	0	0
FG VIE Consolidations		0
FG VIE deconsolidations	0	0
Fair value at end of period	62	62
Change in unrealized gains/(losses) related to financial instruments held	0	0
RMBS [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	334	365
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	7	(2)
Other comprehensive income (loss)	(7)	27
Purchases	0	27
Settlements	(20)	(15)
FG VIE Consolidations		0
FG VIE deconsolidations	0	0
Fair value at end of period	314	402
Change in unrealized gains/(losses) related to financial instruments held	(6)	27
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	787	805
Acquisitions		7
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	15	74
Other comprehensive income (loss)	3	7
Purchases	9	57
Settlements	(5)	(348)
FG VIE Consolidations		0
FG VIE deconsolidations	0	0
Fair value at end of period	809	602
Change in unrealized gains/(losses) related to financial instruments held	4	73
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	(627)	(807)
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	0	(9)
Other comprehensive income (loss)	(2)	0
Purchases	0	0
Settlements	30	44
FG VIE consolidations		0
FG VIE deconsolidations	1	51
Fair value at end of period	(598)	(721)
Change in unrealized gains/(losses) related to financial instruments held	(3)	(7)
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	(130)	(151)
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	1	(2)
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	3	4
FG VIE consolidations		(21)
FG VIE deconsolidations	16	36
Fair value at end of period	(110)	(134)
Change in unrealized gains/(losses) related to financial instruments held	1	(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	(269)	(389)
Acquisitions		0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	34	54
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	(1)	(15)
FG VIE consolidations		0
PG VIE deconsolidations	0	0
Fair value at end of period	(236)	(350)
Change in unrealized gains/(losses) related to financial instruments held as of June 30, 2017	$ 28	$ 25

Fair Value, Measured with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Consolidation

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Fair Value, Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquired In Business Combination

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This element represents consolidations which have taken place during the period in relation to assets measured at fair value on a recurring basis using unobservable inputs (Level 3).

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Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis Asset, Deconsolidations

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Deconsolidation

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Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of unrealized gain (loss) recognized in the income statement for a financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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