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Share-based Compensation and Non-Qualified Deferred Compensation Plan (Narrative) (Details)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
shares
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
shares
Jun. 30, 2017
USD ($)
shares
May 17, 2018
shares
Employee Share Purchase Program maximum shares available 500,000   500,000    
Percentage Of Market Value at Which Shares Purchased Under Employee Share Purchase Plan     85.00%    
Options exercised during the period     26,114    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross     9,994    
Total compensation cost for option and share awards | $ $ 4,300,000 $ 4,700,000 $ 8,800,000 $ 9,500,000  
Total capitalized compensation cost for option and share awards | $ 800,000 $ 900,000 1,600,000 $ 1,900,000  
Employee Share Purchase Plan, Maximum Authorized Amount Per Participant | $     25,000    
Employee Stock Option [Member]          
Allocated Share-based Compensation Expense | $     $ 37,000    
Options exercised during the period     26,000 39,000  
Intrinsic value of options exercised during the period | $     $ 1,300,000 $ 2,000,000  
Remaining weighted average contractual life (years) exercisable options     7 months    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross     10,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term     7 months    
Share Awards and Vesting [Member]          
Total unrecognized compensation cost which is expected to be amortized | $ $ 24,600,000   $ 24,600,000    
Expected amortized period of unrecognized compensation expected to be recognized for share-based compensation plans     3 years    
Fair value of shares vested | $     $ 23,900,000 $ 23,000,000  
Minimum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage     20.00%    
Minimum [Member] | Share Awards and Vesting [Member]          
Vesting period, years     3 years    
Maximum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage     33.33%    
Maximum [Member] | Share Awards and Vesting [Member]          
Vesting period, years     5 years    
2018 Share Incentive Plan          
Total common shares available 8,200,000   8,200,000   9,700,000
New common shares available under the 2018 Share Plan         7,600,000
Shares transferred from the 2011 Share Plan to the 2018 Share Plan         600,000
Common shares subject to awards outstanding under 2011 Share Plan         400,000
Common shares subject to options under the 2011 share plan         100,000
Common shares subject to 2011 Share Plan awards 500,000   500,000    
Shares To Full Value Award Conversion Ratio 3.45        
Value Of Option Right Or Other Award In The Fungible Unit Conversion 1.00        
Full Value award in the share conversion ratio 1.0        
Shares which could be granted pursuant to full value awards 2,400,000   2,400,000