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Impairment of Investment Securities - Additional Information (Detail) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) Bank Security | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Other than temporary impairment charges recognized | $ 0 | $ 0 |
Amortized Cost | 829,188,000 |  | $ 740,846,000
Securities available for sale, at fair value | 812,877,000 |  | 731,358,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 31,725,000 |  | 18,534,000
Number of banks and other financial institutions comprising the security | Bank | 164 |  |
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 | 4 |  |
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 | $ 24,400,000 |  | 29,837,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total | 155 |  |
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% |  |
Probabilities for performing collateral range | 0.33% |  |
Excess present value of future cash flows over our current book value | 19.00% |  |
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 | 114.00% |  |
Probabilities for performing collateral range | 50.00% |  |
Excess present value of future cash flows over our current book value | 101.00% |  |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Percent of Unrealized Losses | 2.00% |  |
Securities available for sale, at fair value | $ 14,132,000 |  | 23,646,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 722,000 |  | 4,379,000
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% |  |
Corporate Debt Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Securities available for sale, at fair value | $ 21,226,000 |  | 16,493,000
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
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 |  |
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 |  |
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 |  |
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 |  |
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 | 83.00% |  |
Reported Value Measurement [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Securities available for sale, at fair value | $ 812,877,000 |  | $ 731,358,000
Federal Home Loan Bank Stock | $ 24,400,000 |  |

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