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Indebtedness (Mortgage Loans and Unsecured Notes) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Aggregate notional amount $ 98.9 $ 101.4
Mortgages [Member]    
Debt Instrument [Line Items]    
Maturity date range, start Jan. 01, 2017  
Maturity date range, end Dec. 31, 2033  
Net book value of collateralized real estate properties $ 540.9  
Fixed Rate Debt [Member]    
Debt Instrument [Line Items]    
Percentage bearing fixed interest, amount 2,569.9  
Aggregate notional amount $ 98.9  
Fixed Rate Debt [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Effective interest rate 3.00%  
Fixed Rate Debt [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Effective interest rate 6.40%  
Mortgage Loans and Unsecured Notes [Member]    
Debt Instrument [Line Items]    
Weighted average remaining life 6 years 8 months