Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 945,000,000	$ 489,000,000
Debt Securities, Available-for-sale, Amortized Cost	2,972,000,000	3,187,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	113,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	921,000,000	401,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	18,000,000	4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,000,000	88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20,000,000	5,000,000
Available-for-sale Securities, Current	3,065,000,000	3,314,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	3,065,000,000
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	75,000,000	201,000,000
Debt Securities, Available-for-sale, Amortized Cost	691,000,000	653,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	75,000,000	201,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	692,000,000	653,000,000
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	870,000,000	288,000,000
Debt Securities, Available-for-sale, Amortized Cost	2,126,000,000	2,450,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	112,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	846,000,000	200,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	18,000,000	4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,000,000	88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20,000,000	5,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	2,218,000,000	2,577,000,000
Fixed Income Securities [Member] \| Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	123,000,000	23,000,000
Debt Securities, Available-for-sale, Amortized Cost	317,000,000	194,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	17,000,000	22,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	114,000,000	13,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	8,000,000	10,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	3,000,000	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	330,000,000	215,000,000
Fixed Income Securities [Member] \| Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	508,000,000	68,000,000
Debt Securities, Available-for-sale, Amortized Cost	1,010,000,000	1,396,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	34,000,000	36,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	508,000,000	63,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7,000,000	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	5,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7,000,000	2,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	1,037,000,000	1,430,000,000
Fixed Income Securities [Member] \| Foreign Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position		20,000,000
Debt Securities, Available-for-sale, Amortized Cost	74,000,000	44,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,000,000	1,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		20,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	75,000,000	44,000,000
Fixed Income Securities [Member] \| US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	7,000,000	38,000,000
Debt Securities, Available-for-sale, Amortized Cost	118,000,000	157,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,000,000	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	7,000,000	36,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	2,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	123,000,000	156,000,000
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	70,000,000	5,000,000
Debt Securities, Available-for-sale, Amortized Cost	258,000,000	200,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	34,000,000	47,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	70,000,000	5,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,000,000	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	291,000,000	248,000,000
Fixed Income Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	0	49,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	50,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	0	0
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	73,000,000	116,000,000
Debt Securities, Available-for-sale, Amortized Cost	74,000,000	147,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	60,000,000	53,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	13,000,000	63,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,000,000	1,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	73,000,000	146,000,000
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	90,000,000	10,000,000
Debt Securities, Available-for-sale, Amortized Cost	275,000,000	263,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20,000,000	24,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	87,000,000	2,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,000,000	7,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,000,000	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7,000,000	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	288,000,000	287,000,000
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,817,000,000	3,103,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	132,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	113,000,000	5,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20,000,000
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	2,910,000,000	3,230,000,000
Collateral Pledged [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		0
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)		85,000,000
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	155,000,000	85,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	155,000,000	85,000,000
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	155,000,000	85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Fixed income securities, at fair value (amortized cost of $2,126 and $2,450)	155,000,000
Asset Pledged as Collateral [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	155,000,000	$ 85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Ambac LSNI [Member] \| Fixed Income Securities [Member] \| Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	467,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 6,000,000

Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ References

No definition available.

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