Investments (Securities Lending) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 3,791	$ 6,642
Security collateral on deposit from counterparties	29	27
Reinvestment portfolio — estimated fair value	3,823	6,571
Amortized cost
Securities Financing Transaction [Line Items]
Securities loaned	3,085	5,895
Estimated fair value
Securities Financing Transaction [Line Items]
Securities loaned	$ 3,748	$ 6,555

This element represents the estimated fair value of the securities lending reinvestment portfolio.

No definition available.

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- Definition

Line items represent financial 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

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 405
-Section 55
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-URI http://asc.fasb.org/extlink&oid=35710563&loc=d3e44140-110984

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-Publisher FASB
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-Section 50
-Paragraph 3
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 05
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=64836671&loc=d3e93645-111703

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- Definition

Carrying amount of liabilities associated with financial assets accounted for as secured borrowings recognized in the transferor's statement of financial position at the end of each period presented.

+ References

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