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Retirement Savings Plan (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Retirement Savings Plan |  |  |
Maximum contribution by participants (as a percent) | 60.00% |  |
Employer matching contributions (as a percent) | 50.00% |  |
Maximum percentage of employees' gross pay subject to employer matching contributions | 4.00% |  |
Benefit expense for the 401(k) Plan | $ 0.5 | $ 0.4 | $ 0.4

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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 + Details
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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. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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