Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 1,217,000,000	$ 489,000,000
Debt Securities, Available-for-sale, Amortized Cost	3,038,000,000	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	81,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,169,000,000	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	77,000,000	4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	48,000,000	88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	81,000,000	5,000,000
Available-for-sale Securities, Current	3,037,000,000	3,314,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	3,037,000,000
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	50,000,000	201,000,000
Debt Securities, Available-for-sale, Amortized Cost	586,000,000	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	50,000,000	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 $2,367 and $2,450)	586,000,000	653,000,000
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	1,167,000,000	288,000,000
Debt Securities, Available-for-sale, Amortized Cost	2,367,000,000	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	81,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,119,000,000	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	77,000,000	4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	48,000,000	88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	81,000,000	5,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,367,000,000	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	23,000,000	23,000,000
Debt Securities, Available-for-sale, Amortized Cost	196,000,000	194,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	23,000,000	22,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	15,000,000	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	1,000,000	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	1,000,000	0
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	218,000,000	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	769,000,000	68,000,000
Debt Securities, Available-for-sale, Amortized Cost	1,284,000,000	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	18,000,000	36,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	769,000,000	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	42,000,000	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	42,000,000	2,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	1,261,000,000	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	1,000,000	20,000,000
Debt Securities, Available-for-sale, Amortized Cost	41,000,000	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	1,000,000	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	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 $2,367 and $2,450)	42,000,000	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	3,000,000	38,000,000
Debt Securities, Available-for-sale, Amortized Cost	168,000,000	157,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	10,000,000	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,000,000	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 $2,367 and $2,450)	177,000,000	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	125,000,000	5,000,000
Debt Securities, Available-for-sale, Amortized Cost	207,000,000	200,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14,000,000	47,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	125,000,000	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	19,000,000	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	19,000,000	0
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	203,000,000	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	51,000,000	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
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,000,000	50,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)		0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	132,000,000	116,000,000
Debt Securities, Available-for-sale, Amortized Cost	144,000,000	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	99,000,000	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	8,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	33,000,000	63,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11,000,000	1,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	132,000,000	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	277,000,000	263,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14,000,000	24,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	83,000,000	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	7,000,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	7,000,000	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	8,000,000	0
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	283,000,000	287,000,000
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,953,000,000	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	81,000,000	5,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	81,000,000
Fixed income securities, at fair value (amortized cost of $2,367 and $2,450)	2,952,000,000	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 $2,367 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	85,000,000	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 $2,367 and $2,450)	85,000,000	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	85,000,000	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 $2,367 and $2,450)	85,000,000
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	85,000,000	$ 85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Non-investment Grade [Member] | Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	12,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,000,000
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	456,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 24,000,000