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Securities Available-for-Sale - Additional Information (Details) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Number of debt securities on which unrealized losses were primarily attributable to changes in interest	51
Number of marketable equity securities in unrealized loss position	52	31
Carrying values of pledged securities | $	$ 249	$ 342
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Number of marketable equity securities in unrealized loss position	1	0

X

- Definition

Number of debt securities on which unrealized losses were primarily attributable to changes in interest.

+ References

No definition available.

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for FHLB debt obligations, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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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.

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