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Summary of Significant Accounting Policies - Stock-Based Compensation (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-based compensation costs      
Time period of historical volatility of stock used to estimate expected volatility 5 years 3 months 18 days    
Weighted-average assumptions used in the Black-Scholes option-pricing model      
Risk-free interest rate (as a percent) 1.97% 1.20% 1.54%
Expected volatility (as a percent) 30.66% 31.23% 36.68%
Dividend yield (as a percent) 0.00% 0.00% 0.00%
Weighted-average expected life 5 years 3 months 18 days 5 years 4 months 24 days 5 years 5 months 12 days