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Income Taxes - Effective income tax reconciliation (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Effective Income Tax Rate, Tax Rate Reconciliation      
Statutory rate 35.00% 35.00% 35.00%
Effect of:      
State and local income taxes, net of federal tax benefit 2.20% 3.00% 3.50%
Excess tax benefits from stock-based compensation 7.10% (1.80%) 0.00%
Nondeductible expenses (4.60%) 1.40% (1.70%)
Reserve for uncertain tax positions 3.60% (0.80%) 2.20%
Nondeductible goodwill impairment 0.00% 0.00% (6.80%)
U.S. federal statutory rate change 13.20% 0.00% 0.00%
Other 6.00% (1.10%) 1.70%
Effective income tax rate 62.50% 35.70% 33.90%