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Share-based Compensation and Non-Qualified Deferred Compensation Plan (Narrative) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
shares
Mar. 31, 2017
USD ($)
shares
Jun. 30, 2011
shares
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 | $ $ 4,500 $ 4,700  
Total capitalized compensation cost for option and share awards | $ 900 $ 900  
Employee Stock Option [Member]      
Allocated Share-based Compensation Expense | $ $ 37    
Options exercised during the period 26,000 39,000  
Intrinsic value of options exercised during the period | $ $ 1,300 $ 2,000  
Remaining weighted average contractual life (years) exercisable options 10 months 24 days    
Share Awards and Vesting [Member]      
Total unrecognized compensation cost which is expected to be amortized | $ $ 28,600    
Expected amortized period of unrecognized compensation expected to be recognized for share-based compensation plans 3 years    
Fair value of shares vested | $ $ 22,400 $ 21,500  
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    
2011 Share Incentive Plan [Member]      
Total fungible units available 2,100,000   9,100,000
Units available under the 2011 Share Plan     5,800,000
Fungible units transferred from the 2002 Share Plan to the 2011 Share Plan     3,300,000
Fungible Units To Full Value Award Conversion Ratio     3.45
Full Value award in the fungible unit conversion ratio     1.0
Value of option right or other award in the fungible unit conversion     0.83
Common shares which could be granted pursuant to full value awards 600,000