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Investments (Narrative) (Detail) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) security | Dec. 31, 2017 USD ($) security
Schedule Of Investments [Line Items] |  |
Fair value of securities on deposit with various regulatory authorities | $ | $ 10 | $ 10
Fair value of securities pledged as collateral | $ | $ 348 | $ 353
Weighted average contractual maturity period in years for securities in an unrealized loss position | 11 years | 12 years
Number of securities in unrealized loss position for a continuous 12 month period | security | 120 | 133
Rate that a security's fair value is below book value | 5.00% | 5.00%
Fair Value Below Book Value Greater Than Five Percent [Member] |  |
Schedule Of Investments [Line Items] |  |
Number of securities in unrealized loss position for a continuous 12 month period | security | 43 | 24

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- Definition Disclosure regarding factors used to determine that the impairment of securities not categorized as trading securities (hence equal to available for sale and held to maturity securities) where cost exceeds fair value. This item contains disclosure of the number of investment positions in the investments determined to be temporarily impaired. + References No definition available. + Details
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- Definition Percentage that a security's fair value is below its book value on available-for-sale and held-to-maturity securities. This percentage is used as a severity threshold for disclosing information about securities in an unrealized loss position for a continuous 12 month period or longer. + References No definition available. + Details
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- Definition Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728 + Details
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