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Corporate Indebtedness (Senior Unsecured Notes)(Details)
12 Months Ended
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Oct. 06, 2010
USD ($)
$ / shares
Mar. 20, 2007
$ / shares
Debt disclosures by scheduled maturity date        
Accreted Balance $ 2,319,244,000 $ 2,107,078,000    
Senior unsecured notes        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance 1,410,316,000      
Accreted Balance 1,386,244,000 1,274,078,000    
Senior unsecured notes | 6.00% Senior unsecured notes maturing on March 31, 2016        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance 255,308,000      
Accreted Balance $ 255,296,000 255,250,000    
Coupon Rate (as a percent) 6.00%      
Effective rate (as a percent) 6.00%      
Term (in Years) 10 years      
Senior unsecured notes | 3.00% Senior unsecured notes maturing on October 15, 2017        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance $ 345,000,000      
Accreted Balance $ 321,130,000 309,069,000    
Coupon Rate (as a percent) 3.00%      
Effective rate (as a percent) 3.00%      
Term (in Years) 7 years      
Premium on sale price to calculate exchange price of notes (as a percent)     30.00%  
Exchange price (in dollars per share) | $ / shares     $ 85.81  
Adjusted exchange rate for the debentures (in shares) 12.3416      
Principal amount of debentures, basis for conversion $ 1,000      
Amount of convertible debt recorded in equity     $ 78,300,000  
Debt Instrument, Unamortized Discount 23,900,000      
Senior unsecured notes | 5.00% Senior unsecured notes maturing on August 15, 2018        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance 250,000,000      
Accreted Balance $ 249,810,000 249,744,000    
Coupon Rate (as a percent) 5.00%      
Effective rate (as a percent) 5.00%      
Term (in Years) 7 years      
Senior unsecured notes | 7.75% Senior unsecured notes maturing on March 15, 2020        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance $ 250,000,000      
Accreted Balance $ 250,000,000 250,000,000    
Coupon Rate (as a percent) 7.75%      
Effective rate (as a percent) 7.75%      
Term (in Years) 10 years      
Senior unsecured notes | 4.50% Senior unsecured notes maturing on December 1, 2022        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance $ 200,000,000      
Accreted Balance $ 200,000,000 200,000,000    
Coupon Rate (as a percent) 4.50%      
Effective rate (as a percent) 4.50%      
Term (in Years) 10 years      
Senior unsecured notes | 4.27% Senior unsecured notes maturing on December 17, 2025        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance $ 100,000,000      
Accreted Balance $ 100,000,000 0    
Coupon Rate (as a percent) 4.27%      
Effective rate (as a percent) 4.27%      
Term (in Years) 10 years      
Senior unsecured notes | 3.00% Senior unsecured notes maturing on March 30, 2027        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance $ 10,008,000      
Accreted Balance $ 10,008,000 10,008,000    
Coupon Rate (as a percent) 3.00%      
Effective rate (as a percent) 3.00%      
Term (in Years) 20 years      
Premium on sale price to calculate exchange price of notes (as a percent)       25.00%
Exchange price (in dollars per share) | $ / shares       $ 173.30
Adjusted exchange rate for the debentures (in shares) 5.7952      
Principal amount of debentures, basis for conversion $ 1,000      
Senior unsecured notes | Senior unsecured notes redeemed April 2015        
Debt disclosures by scheduled maturity date        
Unpaid Principal Balance 0      
Accreted Balance $ 0 $ 7,000