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Securities - Additional Information (Detail) $ in Thousands | Sep. 30, 2018 USD ($) Investment | Dec. 31, 2017 USD ($) Investment
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Schedule of Investments [Line Items] |  |  |
Federal Home Loan Bank stock, at cost | $ | [1] | $ 654,939 | $ 603,819
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses | $ |  | $ 8,500 | $ 10,100
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost |  | 6.70% | 8.50%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost | $ |  | $ 127,100 | $ 118,200
Mortgage-Related Securities |  |  |
Schedule of Investments [Line Items] |  |  |
Number of investment securities designated as having a continuous loss position for twelve months or more |  | 6 | 6
Capital trust notes |  |  |
Schedule of Investments [Line Items] |  |  |
Number of investment securities designated as having a continuous loss position for twelve months or more |  | 5 | 5
Municipal bonds |  |  |
Schedule of Investments [Line Items] |  |  |
Number of investment securities designated as having a continuous loss position for twelve months or more |  | 3 | 3
Mutual funds |  |  |
Schedule of Investments [Line Items] |  |  |
Number of investment securities designated as having a continuous loss position for twelve months or more |  | 1 | 1
[1] | Carrying value and estimated fair value are at cost.

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- Definition Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses. + References No definition available. + Details
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- Definition Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Amortized Cost. + References No definition available. + Details
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- Definition Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809 + Details
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