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Income Taxes - Reconciliation of the statutory U.S. federal tax rate to effective rate (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of the statutory U.S. federal tax rate to effective rate      
Statutory U.S. federal tax rate 21.00% 21.00% 34.00%
Tax Cuts and Jobs Act     33.90%
Tax (windfalls) deficiencies on exercise and vesting of equity awards (17.10%) (21.80%) 5.40%
Stock compensation expense (9.80%) (0.20%) 5.30%
State taxes, net of federal benefit 0.10% (0.10%) 5.10%
Permanent items 1.20% 0.50% 4.40%
Provision to return adjustments (1.10%) (0.30%) (9.50%)
Domestic production activities deduction     (7.20%)
Research and development credits (0.80%) (0.80%) (7.20%)
Effective rate (as a percent) (6.50%) (1.70%) 64.20%