Investments - Investments - Securities Lending and Repurchase Agreements (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities Loaned, Collateral, Obligation to Return Securities	$ 0	$ 0
Securities Lending Transactions:
Gross amount of securities on loan	687	922
Gross amount of associated liability for collateral received	704	945
Repurchase agreements:
Gross amount of recognized liabilities for repurchase agreements	165	174
Gross amount of collateral pledged related to repurchase agreements	$ 170	$ 176
Minimum
Securities Sold Under Agreements to Repurchase, Percentage of Fair Value Required to be Paid in Cash at Sale	95.00%

Securities Loaned, Collateral, Obligation to Return

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- Definition

Securities Loaned, Collateral, Obligation to Return Securities

+ References

No definition available.

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- Definition

Securities Sold Under Agreements to Repurchase, Collateral, Obligation to Return

+ References

No definition available.

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- Definition

Securities Sold Under Agreements to Repurchase, Percentage of Fair Value Required to be Paid in Cash at Sale

+ References

No definition available.

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- References

No definition available.

+ Details

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- Definition

Amount, before effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

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- References

No definition available.

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- Definition

Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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