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Parent Company Condensed Financial Statements (Condensed Balance Sheet - Additional Information) (Details) - $ / shares
Dec. 31, 2018
Dec. 31, 2017
Balance sheets    
Common stock, par value (in dollars per share) $ 0.001 $ 0.001
Common stock, shares authorized (in shares) 200,000,000 200,000,000
Common stock, shares issued (in shares) 165,867,587 165,214,770
Treasury stock, shares (in shares) 20,906,224 20,671,710
Parent Company    
Balance sheets    
Common stock, par value (in dollars per share) $ 0.001 $ 0.001
Common stock, shares authorized (in shares) 200,000,000 200,000,000
Common stock, shares issued (in shares) 165,867,587 165,214,770
Treasury stock, shares (in shares) 20,906,224 20,671,710