Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 515,000,000	$ 489,000,000
Debt Securities, Available-for-sale, Amortized Cost	2,914,000,000	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	141,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	490,000,000	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	5,000,000	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	1,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,000,000	5,000,000
Available-for-sale Securities, Current	3,048,000,000	3,314,000,000
Fixed income securities, at fair value	3,048,000,000
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	152,000,000	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	152,000,000	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	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	363,000,000	288,000,000
Debt Securities, Available-for-sale, Amortized Cost	2,176,000,000	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	140,000,000	132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	338,000,000	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	5,000,000	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	1,000,000	1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,000,000	5,000,000
Fixed income securities, at fair value	2,311,000,000	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	70,000,000	23,000,000
Debt Securities, Available-for-sale, Amortized Cost	322,000,000	194,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	29,000,000	22,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	64,000,000	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	6,000,000	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	351,000,000	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	179,000,000	68,000,000
Debt Securities, Available-for-sale, Amortized Cost	1,066,000,000	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	28,000,000	36,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	179,000,000	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	3,000,000	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	3,000,000	2,000,000
Fixed income securities, at fair value	1,091,000,000	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	12,000,000	20,000,000
Debt Securities, Available-for-sale, Amortized Cost	76,000,000	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	12,000,000	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	77,000,000	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	11,000,000	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	4,000,000	2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	11,000,000	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	1,000,000	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	1,000,000	2,000,000
Fixed income securities, at fair value	122,000,000	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	19,000,000	5,000,000
Debt Securities, Available-for-sale, Amortized Cost	251,000,000	200,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	45,000,000	47,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	19,000,000	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	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	296,000,000	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		49,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss		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		50,000,000
Fixed income securities, at fair value		0
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	67,000,000	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	53,000,000	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	15,000,000	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	1,000,000	1,000,000
Fixed income securities, at fair value	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	5,000,000	10,000,000
Debt Securities, Available-for-sale, Amortized Cost	268,000,000	263,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	33,000,000	24,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,000,000	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	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,000,000	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	1,000,000	0
Fixed income securities, at fair value	300,000,000	287,000,000
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,761,000,000	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	141,000,000	5,000,000
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,000,000
Fixed income securities, at fair value	2,896,000,000	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		85,000,000
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
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	152,000,000	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	152,000,000	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	152,000,000
Asset Pledged as Collateral [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	152,000,000	$ 85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0

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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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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