Investments and Cash - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016 USD ($) Person Security	Dec. 31, 2015 USD ($)
Investment [Line Items]
Accrued investment income	$ 98	$ 99
Government agency obligations as a percentage of total mortgage backed securities	52.00%	54.00%
Number of outside managers managing investment portfolio \| Person	4
Number of securities with unrealized losses greater than 10% of book value for 12 months or more \| Security	11
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	$ 26
Assets held-in-trust	300	$ 283
Fair market value of company's pledged securities	$ 308	$ 305
Investments [Member] \| Internally Managed Portfolio [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	13.00%	13.00%
AGL Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust	$ 1,550	$ 1,411

Represents the number of securities which had unrealized losses greater than 10% of book value for 12 months or more.

No definition available.

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Number of Outside Managers Managing Investment Portfolio

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 4
-Article 7

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624258-113959

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=51801978&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 21
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-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13537-108611

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