Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Transactions accounted for as secured borrowings) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	$ 1,737	$ 1,092
Collateral held under securities lending agreements	1,737	1,093
Repurchase Agreements [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	26	58
Maturity Overnight [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	0	(1)
Maturity Overnight [Member] \| Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	828	423
Maturity Overnight [Member] \| U.S. Treasury and agency
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	36	54
Maturity Overnight [Member] \| Foreign Government Debt [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	712	578
Maturity Overnight [Member] \| Corporate securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	0	37
Maturity Overnight [Member] \| Mortgage backed-securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	74	0
Maturity Overnight [Member] \| Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending collateral	$ 87	$ 0

The fair value of collateral received by the entity associated with securities borrowed that must be returned if the counterparty fulfills its obligations. It appears on the balance sheet as a liability and has a corresponding asset that appears on the balance sheet called Securities Received as Collateral.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Amount of secured borrowings in excess of (less than) the liability for repurchase agreements and securities lending transactions, before offset, included in offsetting disclosures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 7
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL34722452-111729

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- Definition

The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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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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