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Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2017
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit Plan The Company maintains a 401(k) savings plan covering substantially all of its employees. Eligible employees may contribute up to 60% of their annual salary through payroll deductions, but not more than the statutory limits set by the Internal Revenue Service. The Company matches employee contributions at the discretion of the Board. During 2017, 2016 and 2015, the Company’s matching contributions totaled $20.2 million, $15.7 million and $11.2 million, respectively.