Employee 401(k) Plan (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee 401(k) Plan
Maximum percentage of pretax salary, which can be deferred by employees	75.00%
Maximum amount of wages, which can be deferred by employees	$ 18,500
Maximum amount of wages, which can be deferred by employees at least 50 years of age	$ 24,500
Minimum age of employees for a specified contribution amount of wages	50 years
Percentage of employee's salary for which the company contributes a matching contribution	50.00%	50.00%	50.00%
Maximum percentage of employee's salary for which the company contributes a matching contribution	8.00%	8.00%	8.00%
Maximum amount of employee's salary for which the company contributes a matching contribution	$ 12,250,000	$ 12,250,000	$ 12,250,000
Amount of employer contribution	$ 300,000	$ 400,000	$ 400,000

No definition available.

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- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

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

+ References

No definition available.

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- Definition

Maximum amount the employee may contribute to a defined contribution plan.

+ References

No definition available.

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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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- Definition

Represents the maximum amount of pretax salary, which can be deferred by participants of the defined contribution plans.

+ References

No definition available.

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- Definition

Represents the maximum amount of wages, which can be deferred by participants with a specified age, for defined contribution plans.

+ References

No definition available.

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- Definition

Represents the maximum percentage of pretax salary, which can be deferred by participants of the defined contribution plans.

+ References

No definition available.

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- Definition

Represents the maximum amount of wages, which can be deferred by participants with a specified age, for defined contribution plans.

+ References

No definition available.

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