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Employment Benefits	12 Months Ended
Employment Benefits	Dec. 31, 2017
Employment Benefits [Abstract]
Employment Benefits	8. Employment Benefits We have an employee 401(k) salary deferral plan, covering all employees. Employees could make contributions by withholding a percentage of their salary up to the IRS annual limit $18,000 and $24,000 in 2017 for employees under 50 years old and employees 50 years old or over, respectively. We made approximately $3.0 million, $1.7 million and $1.5 million in matching contributions for the years ended December 31, 2017, 2016 and 2015, respectively.