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Secured and unsecured senior debt Unsecured senior notes payable (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Nov. 30, 2015
Jul. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]          
Unsecured senior notes payable     $ 2,046,578 $ 1,747,370  
Repayments of Unsecured Debt     25,000 125,000 $ 250,000
Gains (Losses) on Extinguishment of Debt     $ (189) $ (525) $ (1,992)
Ownership interest in subsidiary (as a percent)     100.00%    
Senior Notes | 4.30% unsecured senior notes payable          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount $ 300,000        
Stated interest rate (as a percent)   4.30%      
Issuance price as percentage of principal amount (percent) 99.624%        
Yield rate (as a percent) 4.345%        
Senior Notes | 2.75% unsecured senior notes payable          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount   $ 400,000      
Stated interest rate (as a percent)   2.75%      
Senior Notes | 4.50% unsecured senior notes payable          
Debt Instrument [Line Items]          
Debt Instrument, Face Amount   $ 300,000      
Stated interest rate (as a percent)   4.50%      
Senior Notes | 2016 Unsecured Senior Bank Term Loan          
Debt Instrument [Line Items]          
Repayments of Unsecured Debt   $ 125,000      
Senior Notes | Line of Credit          
Debt Instrument [Line Items]          
Repayments of Unsecured Debt $ 296,400 $ 569,000