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Impairment of Investment Securities - Additional Information (Detail) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) Security Bank | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) Security Bank | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Other than temporary impairment charges recognized |  |  | $ 0 | $ 0 |
Amortized Cost | $ 784,875,000 |  | 784,875,000 |  | $ 790,082,000
Securities available for sale, at fair value | 778,644,000 |  | 778,644,000 |  | 778,612,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 15,251,000 |  | $ 15,251,000 |  | 22,501,000
Number of banks and other financial institutions comprising the security | Bank | 250 |  | 250 |  |
Number Of Pooled Securities with No Senior Class | Security |  |  | 2 |  |
Number of securities with no excess subordination | Security | 2 |  | 2 |  |
Number of Securities with Excess Subordination | Security | 6 |  | 6 |  |
Coupon rate |  |  | 7.00% |  |
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% |  |
Federal Home Loan Bank Stock | $ 32,302,000 |  | $ 32,302,000 |  | 36,498,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total | 73 |  | 73 |  |
Minimum [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Subordinated tranches range | 7.00% |  | 7.00% |  |
Total principal amount of the respective securities | 5.00% |  | 5.00% |  |
Excess subordination as a percentage of current performing collateral |  |  | 2.00% |  |
Probabilities for performing collateral range |  |  | 0.33% |  |
Excess present value of future cash flows over our current book value |  |  | 18.00% |  |
Maximum [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Subordinated tranches range | 35.00% |  | 35.00% |  |
Excess subordination as a percentage of current performing collateral |  |  | 65.00% |  |
Probabilities for performing collateral range |  |  | 75.00% |  |
Excess present value of future cash flows over our current book value |  |  | 108.00% |  |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Percent of Unrealized Losses | 42.00% |  | 42.00% |  |
Securities available for sale, at fair value | $ 34,629,000 |  | $ 34,629,000 |  | 33,292,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 6,481,000 |  | 6,481,000 |  | 7,124,000
Trust preferred collateralized debt obligations [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Other than temporary impairment charges recognized |  |  | $ 0 | $ 0 |
Percentage of Current Performing Collateral | 0.00% |  | 0.00% |  |
Corporate Debt Securities [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Securities available for sale, at fair value | $ 16,578,000 |  | $ 16,578,000 |  | 6,319,000
Pooled Trust Preferred Securities [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Other than temporary impairment charges recognized | 0 | $ 0 |  |  |
Equity Securities [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Other than temporary impairment charges recognized | 0 | $ 0 |  |  |
Continuous Unrealized Loss Position, Fair Value | 0 |  | 0 |  | 0
Asset Size and Coupon Rate 1 [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Collateral issued by financial institutions | 15,000,000,000 |  | $ 15,000,000,000 |  |
Coupon rate |  |  | 7.00% |  |
Prepayment rate |  |  | 100.00% |  |
Asset Size and Coupon Rate 2 [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Coupon rate |  |  | 7.00% |  |
Fair Value Inputs , Prepayment Rate Year One and Two |  |  | 40.00% |  |
Fair Value Inputs , Prepayment Rate After Year Two |  |  | 2.00% |  |
Asset Size and Coupon Rate 3 [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Prepayment rate |  |  | 5.00% |  |
Asset Size and Coupon Rate 3 [Member] | Minimum [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Collateral issued by financial institutions | 2,000,000,000 |  | $ 2,000,000,000 |  |
Asset Size and Coupon Rate 3 [Member] | Maximum [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Collateral issued by financial institutions | 15,000,000,000 |  | 15,000,000,000 |  |
Asset Size and Coupon Rate 4 [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Collateral issued by financial institutions | $ 2,000,000,000 |  | $ 2,000,000,000 |  |
Coupon rate |  |  | 10.00% |  |
Prepayment rate |  |  | 5.00% |  |
Asset Size and Coupon Rate 5 [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Prepayment rate |  |  | 0.00% |  |
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Percent of Unrealized Losses | 50.00% |  | 50.00% |  |
Reported Value Measurement [Member] |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |
Securities available for sale, at fair value | $ 778,644,000 |  | $ 778,644,000 |  | $ 778,612,000
Federal Home Loan Bank Stock | $ 32,302,000 |  | $ 32,302,000 |  |

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