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Real Estate (Disposition of Ground Lease Business) (Details)
 1 Months Ended 12 Months Ended
Apr. 14, 2017
USD ($)
Apr. 30, 2017
USD ($)
lease
property
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
lease_asset
property
Dec. 31, 2015
USD ($)
Mar. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Number of properties acquired | property       2    
Number of net lease assets | lease_asset       1    
Net proceeds from sales of real estate     $ 314,013,000 $ 435,560,000 $ 362,530,000  
Income from sales of real estate     $ 92,049,000 $ 105,296,000 $ 93,816,000  
Safety, Income and Growth, Inc.            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Carrying value of properties   $ 161,100,000        
Income from sales of real estate   $ 123,400,000        
Ground Net Lease Business | Safety, Income and Growth, Inc.            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Number of properties acquired | property   12        
Number of net lease assets | lease   7        
Number of master lease assets | lease   1        
Number of properties covered under master lease agreement | property   5        
Net proceeds from sales of real estate $ 113,000,000 $ 113,000,000        
Payments to acquire equity method investments $ 55,500,000 $ 55,500,000        
2017 Secured Financing            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Debt instrument, face amount           $ 227,000,000.0