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Share-Based Compensation and Benefit Plans (Narrative) (Details)
 3 Months Ended 12 Months Ended
Jun. 30, 2011
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | shares   50,710 202,993 53,358
Granted, Options Outstanding | shares   14,622 12,854 26,752
Total compensation cost for option and share awards | $   $ 18,800,000 $ 21,300,000 $ 18,600,000
Total capitalized compensation cost for option and share awards | $   $ 3,800,000 $ 3,800,000 3,500,000
Percentage Of Market Value Per Share At Which Shares Purchased Under Employee Share Purchase Plan   85.00%    
Employee Share Purchase Plan, Maximum Authorized Amount Per Participant | $   $ 25,000    
Share Awards Held In Employee Trust Shares | shares   1,700,000 1,800,000  
Share Awards Held In Non Qualified Deferred Compensation Plan Shares | $   $ 26,700,000 $ 34,500,000  
Cash Required By Plan Participants Upon Withdrawal | $   $ 22,300,000 $ 22,700,000  
Share Awards Held In Non Qualified Deferred Compensation Plan Shares | shares   1,000,000 1,100,000  
Trading Securities Held In Non Qualified Deferred Compensation Plan Value | $   $ 93,600,000 $ 59,500,000  
Defined Contribution Plan, Cost | $   $ 2,700,000 $ 2,700,000 $ 2,600,000
Number of shares diversified | shares   261,000 297,000  
Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | shares   100,000 200,000 100,000
Total intrinsic value of options exercised during the period | $   $ 2,200,000 $ 8,900,000 $ 2,000,000
Remaining weighted average contractual life (years) outstanding options   1 year 1 month    
Remaining weighted average contractual life (years) exercisable options   1 year 1 month    
Allocated Share-based Compensation Expense | $   $ 100,000 100,000 200,000
Share Awards and Vesting [Member]        
Total unrecognized compensation cost related to unvested options which is expected to be amortized | $   19,900,000    
Fair value of shares vested | $   $ 23,100,000 $ 27,200,000 $ 19,200,000
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    
2011 Share Incentive Plan [Member]        
Total fungible units to be issued | shares 9,100,000 2,900,000    
Units available under the 2011 Share Plan | shares 5,800,000      
Fungible units transferred from the 2002 Share Plan to the 2011 Share Plan | shares 3,300,000      
Fungible Units To Full Value Award Conversion Ratio 3.45      
Full Value award in the fungible unit conversion ratio | shares 1.0      
Expiration value of option right or other award 0.83      
Common shares which could be granted pursuant to full value awards | shares   800,000