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401(k) Plan (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016
Retirement Benefits [Abstract]
Employee contribution, percentage of annual salary		100.00%
Employee contribution, maximum allowable contribution		$ 55,000
Service period for eligibility (in years)	1 month		1 year
Number of hours of service required in 12 months for plan eligibility (in hours)		1000 hours
Employer matching contribution percent		100.00%
Percent of employees' gross pay		4.00%
Term for employees to become fully vested (in years)		6 years
Profit sharing contribution, percent		7.00%
Company contributions to plan		$ 5,910,000		$ 6,433,000	$ 7,600,000

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Amount of cost for defined contribution plan.

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

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