Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains (Losses)	$ 17,805	$ 14,342
Securities available for sale	909,247	731,358
Amortized Cost	924,054	740,846
Held-to-maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	250,300	338,300
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	636,300	569,000
Sales [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Losses	4,719	359	$ 305
Available-for-sale Securities, Gross Realized Losses	0	316	277
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	4,719	43	28
Calls And Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Gains	3,383	5,057	589
Available-for-sale Securities, Gross Realized Losses	0	60	0
Gain Losses Maturities And Impairment	$ 3,383	4,997	$ 589
Pre Tsl Xiii [Member] \| Calls And Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Gains		4,300
Pre TSL VII [Member] \| Calls And Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Gains		$ 700

Gain losses maturities and impairment.

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This item represents the gross gain on maturities of available for sale securities.

+ References

No definition available.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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No definition available.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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+ References

No definition available.

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The carrying amount as of the date of the latest financial statement presented of other securities classified as available for sale or held for investment which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, 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://fasb.org/us-gaap/role/ref/legacyRef
-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=109249958&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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