Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation (Narrative) (Details)

v3.8.0.1
Stock-Based Compensation (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options $ 10,463,000 $ 9,576,000 $ 11,872,000
Shares authorized 20,800,000    
Shares available for future grants 2,701,396    
Stock-based compensation expense $ 11,286,000 10,986,000 11,672,000
Stock Options [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Stock-based compensation expense 3,843,000 3,896,000 3,637,000
Unrecognized costs related to share based compensation $ 7,166,000    
Weighted average period of recognition of unrecognized costs related to stock options, years 2 years 4 months 6 days    
Intrinsic value of stock options exercised $ 19,836,000 12,976,000 20,400,000
Cash received on exercises of stock options $ 19,946,000 13,167,000 17,442,000
Stock Options [Member] | After2008 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 4 years    
Expiration period, years 10 years    
Service Based Restricted Stock Units [Member] | After2008 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 4 years    
Performance Based Restricted Stock Units [Member] | Minimum [Member] | Prior To2010 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 2 years 6 months    
Performance Based Restricted Stock Units [Member] | Minimum [Member] | After2009 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 2 years    
Performance Based Restricted Stock Units [Member] | Maximum [Member] | Prior To2010 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 3 years    
Performance Based Restricted Stock Units [Member] | Maximum [Member] | After2009 [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Vesting period of options granted, years 3 years    
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]      
Stock-based compensation expense $ 7,443,000 $ 7,090,000 $ 8,035,000
Unrecognized costs related to share based compensation $ 14,174,000    
Weighted average period of recognition of unrecognized costs related to stock options, years 2 years 2 months 26 days    
Company withheld shares 65,437 88,335 90,355
Company withheld shares, tax obligations $ 3,662,000 $ 5,127,000 $ 4,895,000
Grant date fair value of vested units $ 7,477,000 $ 7,988,000 $ 6,848,000