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Securities - Additional Information (Detail) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2017 USD ($) Investment | Dec. 31, 2016 USD ($) Investment
Schedule of Investments [Line Items] |  |  |  |
Federal Home Loan Bank stock, at cost | [1] | $ 589,067 | $ 590,934 |
Amortized cost from sales of held to maturity securities |  | 521,000 | 3,721,368 | [2]
Proceeds from sales of held to maturity securities |  | 547,925 |  |
Gross realized gains from sales of held to maturity securities |  | 26,900 |  |
Securities transferred from held to maturity to available for sale |  | 3,040,305 |  |
Unrealized gain on securities transferred from held to maturity to available for sale |  | 82,800 |  |
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses |  | $ 11,300 | $ 14,100 |
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost |  | 25.90% | 32.20% |
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost |  | $ 43,700 | $ 43,700 |
Capital trust notes |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Number of investment securities designated as having a continuous loss position for twelve months or more | Investment |  | 5 | 5 |
[1] | Carrying value and estimated fair value are at cost.
[2] | Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).

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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://www.xbrl.org/2003/role/presentationRef -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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- Definition This item represents the gain (loss) realized on the sale of held-to-maturity securities disposed of during the period. Additionally, this item may include, if not separately presented, any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt securities which are being or have been recognized in earnings during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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