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Stock Incentive Plans - Summary of Share-based Awards Available for Grant under 2016 Plan (Details) - 2016 Plan - shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Awards Available for Grant, Beginning balance 4,717,292 4,034,152
Additional awards authorized 1,218,402 1,168,865
Awards Available for Grant, Awards granted (481,366) (666,913)
Awards Available for Grant, Awards forfeited 46,222 124,478
Awards withheld for tax purposes 33,929 56,710
Awards Available for Grant, Ending balance 5,534,479 4,717,292