Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Parenthetical) (Details) - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred Stock, Shares Issued	0	0
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001	$ 0.001
Common Stock, Shares Authorized	75,000,000	75,000,000
Common Stock, Shares, Issued	65,439,245	64,917,683
Treasury stock (in shares)	24,721,000	24,672,932	23,971,522	23,308,430
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred Stock, Shares Issued	0	0
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	75,000,000	75,000,000
Common Stock, Shares, Issued	65,439,245	64,917,683
Treasury stock (in shares)	24,721,372	24,672,932