Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 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
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	$ 876	1,261
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	17	(4)
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	(46)	(66)
FG VIE Consolidations	21
FG VIE deconsolidations	(87)	0
Fair value at end of period	781	1,191
Change in unrealized gains/(losses) related to financial instruments held	21	4
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	65	65
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(2)	(16)
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	63	49
Change in unrealized gains/(losses) related to financial instruments held	(2)	(16)
Obligations of state and political subdivisions [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	39	8
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	0
Other comprehensive income (loss)	4	0
Purchases	0	0
Settlements	(2)	(1)
FG VIE Consolidations	0
FG VIE deconsolidations	0	0
Fair value at end of period	42	7
Change in unrealized gains/(losses) related to financial instruments held	4	0
Corporate securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	60	71
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	2	2
Other comprehensive income (loss)	0	1
Purchases	0	0
Settlements	0	0
FG VIE Consolidations	0
FG VIE deconsolidations	0	0
Fair value at end of period	62	74
Change in unrealized gains/(losses) related to financial instruments held	0	1
RMBS [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	365	348
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(2)	(2)
Other comprehensive income (loss)	27	(5)
Purchases	27	34
Settlements	(15)	(15)
FG VIE Consolidations	0
FG VIE deconsolidations	0	0
Fair value at end of period	402	360
Change in unrealized gains/(losses) related to financial instruments held	27	(6)
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	805	657
Acquisitions	7
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	74	1
Other comprehensive income (loss)	7	(5)
Purchases	57	0
Settlements	(348)	(14)
FG VIE Consolidations	0
FG VIE deconsolidations	0	0
Fair value at end of period	602	639
Change in unrealized gains/(losses) related to financial instruments held	73	(5)
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	(807)	(1,225)
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(9)	21
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	44	39
FG VIE consolidations	0
FG VIE deconsolidations	51	0
Fair value at end of period	(721)	(1,165)
Change in unrealized gains/(losses) related to financial instruments held	(7)	21
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	(151)	(124)
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	(2)	2
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	4	3
FG VIE consolidations	(21)
FG VIE deconsolidations	36	0
Fair value at end of period	(134)	(119)
Change in unrealized gains/(losses) related to financial instruments held	(2)	1
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	(389)	(365)
Acquisitions	0
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	54	(60)
Other comprehensive income (loss)	0	0
Purchases	0	0
Settlements	(15)	(9)
FG VIE consolidations	0
FG VIE deconsolidations	0	0
Fair value at end of period	(350)	(434)
Change in unrealized gains/(losses) related to financial instruments held	$ 25	$ (79)

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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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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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 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 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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