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This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for those securities which are categorized neither as held-to-maturity nor trading securities and which have unrealized losses greater than 10% of book value.

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Represents government agency obligations as a percentage of the aggregate amortized cost of mortgage backed securities.

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Number of Outside Managers Managing Investment Portfolio

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Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

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The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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The minimum amount the entity agreed to spend under the long-term purchase commitment.

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Investments and Cash - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
Investments and Cash - Narrative (Details) $ in Millions	Feb. 28, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security manager	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Security
Investment [Line Items]
Proceeds from sale of fixed-maturity securities available-for-sale		$ 316	$ 350	$ 908	$ 1,128
Accrued investment income		$ 98		$ 98		$ 97
Government agency obligations as a percentage of total mortgage backed securities		47.00%		47.00%		39.00%
Number of securities with unrealized losses greater than 10% of book value for 12 months or more \| Security		35		35		28
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more		$ 49		$ 49		$ 27
Assets held-in-trust		262		$ 262		287
Number of outside managers managing investment portfolio \| manager				7
Effect of adoption of ASU						$ 32
Future Equity Investments [Member]
Investment [Line Items]
Long-term purchase commitment, amount	$ 100
Investments [Member] \| Internally Managed Portfolio [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar				12.00%		12.00%
AGL Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust		1,725		$ 1,725		$ 1,677
Accounting Standards Update 2016-01 [Member] \| Retained Earnings [Member]
Investment [Line Items]
Effect of adoption of ASU						(32)
Redeemable Preferred Stock [Member] \| Accounting Standards Update 2016-01 [Member] \| Retained Earnings [Member]
Investment [Line Items]
Effect of adoption of ASU						$ 1
Other Income [Member]
Investment [Line Items]
Recorded gain on change in fair value of equity securities		$ 32		$ 29