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Employee Benefit Plans - Narrative (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) SERP_retirement_age | Dec. 31, 2017 USD ($) year | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Retirement Benefits [Abstract] |  |  |  |  |  |  |
Employers matching contribution |  |  | 50.00% |  |  |  |
Employees first contribution |  |  | 6.00% |  |  |  |
401(k) profit sharing plan, employers matching contribution amount |  | $ 1,900,000 |  |  |  | $ 1,600,000 | $ 1,100,000
SERP annual retirement benefits period (years) | 15 years |  |  |  |  |  |
SERP retirement age |  |  |  | 65 | 65 |  |
SERP target annual retirement benefit | $ 300,000 | 300,000 | $ 300,000 | $ 300,000 | $ 300,000 |  |
SERP annual rate of return |  |  | 7.00% |  |  |  |
SERP present value of the amount owed | $ 4,600,000 | $ 4,600,000 | $ 4,600,000 | $ 4,600,000 | $ 4,600,000 |  |

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- Definition The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 710 -SubTopic 10 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6409875&loc=d3e20015-108363 + Details
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