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Income Taxes - Reconciliation of Federal Statutory Rate and Effective Rate (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Federal statutory rate 35.00% 35.00% 35.00%
Rate of state income taxes, net of federal tax benefit 4.50% 2.80% 2.00%
Statutory rate other than U.S. statutory rate 0.10% 0.40% 0.00%
Sec. 987 regulation change, federal and state impact (0.00%) 1.20% (0.00%)
U.S. federal research and development credit 3.00% 1.50% 1.00%
Impacts of U.S. federal tax reform 14.20% 0.00% 0.00%
Other, net (1.20%) (0.70%) 1.90%
Effective tax rate 55.60% 40.20% 39.90%