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Employee Savings Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Minimum age to be eligible to defined contribution plan	21 years
Maximum contribution by company expressed as percentage of employee pre-tax salary	50.00%
Company match percent	50.00%
Percent of employee contribution	6.00%
Company contributions vest at end of two years	20.00%
Company contributions vest at end of three years	40.00%
Company contributions vest at end of four years	60.00%
Company contributions vest at end of five years	100.00%
Company contributions to employee savings plan	$ 2,030,000	$ 1,712,000	$ 1,845,000
Highly Compensated Employee
Defined Contribution Plan Disclosure [Line Items]
Maximum contribution by company expressed as percentage of employee pre-tax salary	25.00%

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- Definition

Company contributions vest at end of five years.

+ References

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- Definition

Company contributions vest at end of four years.

+ References

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- Definition

Company contributions vest at end of three years.

+ References

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- Definition

Company contributions vest at end of two year.

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- Definition

Minimum age to be eligible to defined contribution plan.

+ References

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ 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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

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- Definition

Percentage employer matches of the employee's percentage contribution matched.

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

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