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Impairment of Investment Securities - Additional Information (Detail) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) Security Bank | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  | $ 740,846,000 | $ 790,082,000 |
Estimated Fair Value of Available for Sale |  | $ 731,358,000 | 778,612,000 |
Number of banks and other financial institutions comprising the security | Bank |  | 206 |  |
Number Of Pooled Securities with No Senior Class | Security |  | 2 |  |
Number of Securities with No Excess Subordination | Security |  | 2 |  |
Number of Securities with Excess Subordination | Security |  | 5 |  |
Collateral issued by financial institutions |  | $ 15,000,000,000 |  |
Coupon interest rate |  | 7.00% |  |
Prepayment rate |  | 5.00% |  |
Estimate of future cash flows prepayment percentage in year three |  | 40.00% |  |
Estimate of future cash flows prepayment percentage thereafter |  | 2.00% |  |
Fair Value Input Coupon Rates |  | 10.00% |  |
Probability of default |  | 100.00% |  |
Projected recovery rate |  | 0.00% |  |
Probability percentage assigned to default bank subject to market indicators |  | 10.00% |  |
Probability percentage assigned to default bank |  | 100.00% |  |
Other than-temporary impairment charge |  | $ 3,958,000 | 6,671,000 | $ 218,000
Other investments |  | $ 29,837,000 | 36,498,000 |
Minimum [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Subordinated tranches range |  | 7.00% |  |
Total principal amount of the respective securities |  | 5.00% |  |
Excess subordination as a percentage of current performing collateral |  | 2.00% |  |
Collateral issued by financial institutions |  | $ 15,000,000,000 |  |
Probabilities for performing collateral range |  | 0.33% |  |
Excess percent value of future cash flows over current book value |  | 18.00% |  |
Over Fifteen Million [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Prepayment rate |  | 100.00% |  |
Maximum [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Subordinated tranches range |  | 35.00% |  |
Excess subordination as a percentage of current performing collateral |  | 72.00% |  |
Collateral issued by financial institutions |  | $ 2,000,000,000 |  |
Probabilities for performing collateral range |  | 50.00% |  |
Excess percent value of future cash flows over current book value |  | 104.00% |  |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Total unrealized losses |  | 24.00% |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  | $ 27,499,000 | 39,989,000 |
Estimated Fair Value of Available for Sale |  | 23,646,000 | 33,292,000 |
Other than-temporary impairment charge |  | $ 0 | 0 | $ 0
Obligations of U.S. Government-Sponsored Enterprises [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Total unrealized losses |  | 65.00% |  |
Corporate Securities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  | $ 15,907,000 | 5,903,000 |
Estimated Fair Value of Available for Sale |  | $ 16,493,000 | 6,319,000 |
US States and Political Subdivisions Debt Securities [Member] | Other Government - Sponsored Enterprises [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Total unrealized losses |  | 10.00% |  |
Debt Securities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Number of securities in an unrealized loss positions | Security |  | 98 |  |
Estimated Fair Value of Available for Sale |  | $ 729,688,000 | 776,942,000 |
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Number of securities in an unrealized loss positions |  | 0 |  |
Equity Securities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Number of securities in an unrealized loss positions | Security |  | 0 |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  | $ 1,670,000 | 1,670,000 |
Estimated Fair Value of Available for Sale |  | 1,670,000 | 1,670,000 |
Corporate Securities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Estimated Fair Value of Available for Sale |  | $ 16,493,000 | 6,319,000 |
Trust Preferred Collateralized Debt Obligations [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Percentage of Current Performing Collateral |  | 0.00% |  |
Mezzanine [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  | $ 27,499,000 |  |
Estimated Fair Value of Available for Sale |  | 23,646,000 |  |
Pre Tsl Xiii [Member] | Calls And Maturities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Available-for-sale Securities, Gross Realized Losses |  | $ 4,329,000 |  |
Corporate/Mortgage-Backed Securities, Amortized cost |  |  | 13,171,000 |
Pre TSL VII [Member] | Calls And Maturities [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Available-for-sale Securities, Gross Realized Losses | $ 712,000 |  | 589,000 |
Corporate/Mortgage-Backed Securities, Amortized cost |  |  | $ 3,119,000 |

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