Parent Company Condensed Financial Statements (Condensed Balance Sheet - Additional Information) (Details) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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)	165,214,770	164,604,072
Treasury stock, shares (in shares)	20,671,710	20,436,621
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)	165,214,770	164,604,072
Treasury stock, shares (in shares)	20,671,710	20,436,621