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Rental Properties Summary Of Carrying Amounts Of Rental Properties (Details)
12 Months Ended
Apr. 21, 2014
USD ($)
state
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Real Estate Properties [Line Items]        
Real Estate Investment Property Gross   $ 3,559,502,000 $ 2,917,194,000  
Accumulated depreciation   (534,303,000) (465,660,000)  
Total   3,025,199,000 2,451,534,000  
Depreciation expense on rental properties   85,900,000 63,000,000 $ 50,700,000
Noncash or Part Noncash Acquisition, Debt Assumed   0 101,441,000 $ 19,710,000
Building and Building Improvements [Member]        
Real Estate Properties [Line Items]        
Real Estate Investment Property Gross   2,837,611,000 2,273,430,000  
Furniture and Fixtures [Member]        
Real Estate Properties [Line Items]        
Real Estate Investment Property Gross   34,423,000 25,922,000  
Land [Member]        
Real Estate Properties [Line Items]        
Real Estate Investment Property Gross   $ 687,468,000 $ 617,842,000  
immaterial business acquisition [Member]        
Real Estate Properties [Line Items]        
Business Acquisition, Percentage of Voting Interests Acquired 100.00%      
Number of properties acquired 11      
Number of States in which Entity Operates | state 7      
Lessor Leasing Arrangements, Operating Leases, Term of Contract 13 years 20 years    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets $ 123,700,000      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles 3,300,000      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt 101,500,000      
Business Combination, Acquisition Related Costs 500,000      
Mortgage notes payable [Member] | Mortgage notes payable, 4.00%, due July 6, 2017 | immaterial business acquisition [Member]        
Real Estate Properties [Line Items]        
Business Combination, Consideration Transferred 117,700,000      
Noncash or Part Noncash Acquisition, Debt Assumed 90,300,000      
Notes Payable, Other Payables [Member] | Note payable, 2.50%, due April 21, 2016 | immaterial business acquisition [Member]        
Real Estate Properties [Line Items]        
Noncash or Part Noncash Acquisition, Debt Assumed $ 1,900,000