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Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Maturity of contract |  | less than one year |  |
Approximate year of maturity |  | 30 years |  |
Gross Unrealized Gains (Losses) |  | $ 14,342 | $ 18,546 |
Securities available for sale |  | 731,358 | 778,612 |
Amortized Cost |  | 740,846 | 790,082 |
Available-for-sale securities pledged as collateral |  | 569,000 | 445,800 |
Held-to-maturity securities pledged as collateral |  | 338,300 | 119,200 |
Sales [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale Securities, Gross Realized Losses |  | 359 | 305 | $ 0
Available-for-sale Securities, Gross Realized Losses |  | 316 | 277 | 284
Sales [Member] | US Government Agencies Debt Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  |  |  | 75,000
Calls And Maturities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Gross Gains |  | 5,057 | 589 | 131
Available-for-sale Securities, Gross Realized Losses |  | 60 | 0 | $ 0
Calls And Maturities [Member] | Pre TSL VII [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale Securities, Gross Realized Losses | $ 712 |  | 589 |
Gross Gains |  | 700 |  |
Amortized Cost |  |  | 3,119 |
Calls And Maturities [Member] | Pre TSL XIII [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale Securities, Gross Realized Losses |  | $ 4,329 |  |
Amortized Cost |  |  | $ 13,171 |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition Description of how prepayments are considered in determination of contractual cash flows and cash flows expected to be collected for the loan or debt security. This relates to loans and debt securities for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable or all cash flows expected at acquisition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15029-111544 + Details
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