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Employee Benefit Plans (401(k) Savings Plan) - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract] |  |  |
Criteria of employer to contribute in employee saving plan | 100% of an employee’s first 3% of contributions and 50% of their next 2% of contributions |  |
Percentage of employer match to employee's contribution | 100.00% |  |
Percentage of employer match to employee's contribution | 50.00% |  |
Upper limit of employer match | 4.00% |  |
Maximum statutory compensation under code | $ 270,000 |  |
Percentage contributions under the savings plan, vested | 100.00% |  |
Contributions and plan administration costs for savings plan | $ 4,600,000 | $ 4,100,000 | $ 4,700,000

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- Definition Maximum employee earnings amount eligible for matching contributions under IRS regulations. + References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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