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Borrowings (Textual) (Details)
 12 Months Ended
Dec. 31, 2015
USD ($)
Contract
instrument
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Jun. 10, 2014
Debt Instrument [Line Items]        
Notes Payable $ 3,427,568,000 $ 3,512,700,000    
Average interest rate (percentage) 3.70% 3.70%    
Interest Rate Swap        
Debt Instrument [Line Items]        
Debt Instrument, Periodic Payment, Interest $ 200,000,000      
Interest rate, stated percentage 4.17%      
Fixed Rate Debt        
Debt Instrument [Line Items]        
Notes Payable $ 3,148,100,000 $ 3,074,700,000    
Conventional Variable Rate Debt        
Debt Instrument [Line Items]        
Long-term debt $ 65,000,000      
Variable interest rate (percent) 0.80%      
Floating Rate Debt        
Debt Instrument [Line Items]        
Notes Payable $ 265,000,000 397,900,000    
Capped Conventional Variable Rate Debt        
Debt Instrument [Line Items]        
Long-term debt $ 125,000,000      
Variable interest rate (percent) 0.80%      
Other Than Conventional Variable Rate Debt        
Debt Instrument [Line Items]        
Long-term debt $ 1,100,000,000      
Average interest rates (percent) 4.00%      
Fixed Rate Senior Notes        
Debt Instrument [Line Items]        
Unsecured debt $ 2,100,000,000      
Long-term debt fixed interest rate (percent) 3.90%      
Unsecured Debt        
Debt Instrument [Line Items]        
Notes Payable $ 2,141,332,000      
Average interest rate (percentage) 3.80%      
Unsecured Debt | 4.0% Senior Notes due 2025 [Member]        
Debt Instrument [Line Items]        
Debt instrument, unamortized discount $ 4,000,000      
Underwriting commissions and expenses 2,600,000      
Proceeds from Unsecured Notes Payable $ 393,400,000      
Interest rate, stated percentage 4.00%      
Debt Instrument, Issue Price $ 0.9899      
Debt Instrument, Face Amount $ 400,000,000      
Covenant Obligation, Debt to Total Asset Value 60.00%      
Covenant obligation, Secured Debt to Total Asset Value Ratio 40.00%      
Covenant Obligation, Unencumbered Leverage Ratio 150.00%      
Unsecured Debt | $750 million unsecured revolving credit facility [Member]        
Debt Instrument [Line Items]        
Average interest rate (percentage) 1.23%      
Line of credit facility, maximum borrowing capacity $ 750,000,000      
Line of credit facility, amount outstanding 75,000,000      
Unsecured Debt | $500 million unsecured revolving credit faciltiy [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Current Borrowing Capacity 746,700,000      
Credit facilities, commitment fees 1,100,000      
Letters of credit outstanding amount 3,300,000      
Unsecured Debt | Fixed Rate Debt        
Debt Instrument [Line Items]        
Notes Payable $ 1,535,200,000 1,320,200,000    
Average interest rates (percent) 4.21%      
Unsecured Debt | Floating Rate Debt        
Debt Instrument [Line Items]        
Notes Payable [1] $ 75,000,000 59,000,000    
Average interest rates (percent) [1] 1.23%      
Interest rate, stated percentage [1] 1.23%      
Unsecured Debt | KeyBank Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 150,000,000      
Unsecured Debt | Wells Fargo Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount 250,000,000      
Unsecured Debt | US Bank Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 150,000,000      
Unsecured Debt | Term Loan [Member]        
Debt Instrument [Line Items]        
Average interest rate (percentage) 3.10%      
Debt Instrument, Face Amount $ 550,000,000      
Unsecured Debt | 3.75% Seinor Notes due 2024 [Member]        
Debt Instrument [Line Items]        
Covenant Obligation, Debt to Total Asset Value       60.00%
Covenant obligation, Secured Debt to Total Asset Value Ratio       40.00%
Covenant Obligation, Unencumbered Leverage Ratio       150.00%
Unsecured Debt | 2011 and 2012 Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 310,000,000      
Unsecured Debt | Minimum | $750 million unsecured revolving credit facility [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 0.85%      
Unsecured Debt | Minimum | KeyBank Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 0.90%      
Unsecured Debt | Minimum | Wells Fargo Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 0.90%      
Unsecured Debt | Minimum | 3.75% Seinor Notes due 2024 [Member]        
Debt Instrument [Line Items]        
Covenant obligation, Fixed Charge Ratio       100.00%
Unsecured Debt | Maximum | 4.0% Senior Notes due 2025 [Member]        
Debt Instrument [Line Items]        
Covenant obligation, Fixed Charge Ratio 150.00%      
Unsecured Debt | Maximum | $750 million unsecured revolving credit facility [Member]        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity $ 1,500,000,000      
Debt Instrument, Basis Spread on Variable Rate 1.55%      
Unsecured Debt | Maximum | KeyBank Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 1.75%      
Unsecured Debt | Maximum | Wells Fargo Term Loan [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Basis Spread on Variable Rate 1.90%      
Unsecured Debt | Maximum | 3.75% Seinor Notes due 2024 [Member]        
Debt Instrument [Line Items]        
Covenant obligation, Fixed Charge Ratio       150.00%
Unsecured Debt | Variable Rate Debt | $500 million unsecured revolving credit faciltiy [Member]        
Debt Instrument [Line Items]        
Average interest rate (percentage) 1.20%      
Line of credit facility, amount outstanding $ 75,000,000      
Unsecured Debt | Fixed Rate Debt | Senior Notes        
Debt Instrument [Line Items]        
Average interest rate (percentage) 4.20%      
Debt Instrument, Face Amount $ 1,535,246,000      
Unsecured Debt | Fixed Rate Debt | Public Income Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount 1,200,000,000      
Unsecured Debt | Fixed Rate Debt | Private Placement [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 0      
Secured Debt [Member]        
Debt Instrument [Line Items]        
Average interest rate (percentage) 3.50%      
Secured Debt [Member] | FNMA        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity $ 240,000,000      
Line of credit facility, amount outstanding 240,000,000      
Line of Credit Facility, Current Borrowing Capacity $ 240,000,000      
Debt renewal period (days) 90 days      
Debt Instrument, Basis Spread on Variable Rate 0.17%      
Secured credit facilities, commitment fee percentage (percent) 0.62%      
Credit facilities, commitment fees $ 100,000      
Secured Debt [Member] | Fixed Rate Debt        
Debt Instrument [Line Items]        
Notes Payable $ 1,062,862,000      
Average interest rate (percentage) 3.90%      
Secured Debt [Member] | Floating Rate Debt        
Debt Instrument [Line Items]        
Notes Payable [2] $ 190,000,000      
Average interest rate (percentage) 0.80%      
Secured Debt [Member] | Variable Rate Debt | FNMA        
Debt Instrument [Line Items]        
Average interest rate (percentage) 0.80%      
Line of credit facility, amount outstanding $ 190,000,000      
Secured Debt [Member] | Variable Rate Debt | Mortgages        
Debt Instrument [Line Items]        
Gains (Losses) on Extinguishment of Debt $ 200,000      
Secured Debt [Member] | Fixed Rate Debt | FNMA        
Debt Instrument [Line Items]        
Average interest rate (percentage) 4.70%      
Line of credit facility, amount outstanding $ 50,000,000      
Secured Debt [Member] | Fixed Rate Debt | Mortgages        
Debt Instrument [Line Items]        
Average interest rate (percentage) 3.90%      
Mortgage Loans on Real Estate $ 1,012,862,000      
Interest Rate Cap | Secured Debt [Member] | FNMA        
Debt Instrument [Line Items]        
Number of Interest rate derivatives held (contracts) | Contract 5      
Derivative, Notional Amount $ 125,000,000      
Limited Partner [Member]        
Debt Instrument [Line Items]        
Unsecured debt 2,141,332,000 1,923,058,000    
Proceeds from notes payable 395,960,000 396,855,000 $ 347,759,000  
Gains (Losses) on Extinguishment of Debt $ (3,602,000) $ (2,586,000) $ (426,000)  
Scenario 1 [Member] | Interest Rate Cap | Secured Debt [Member] | FNMA        
Debt Instrument [Line Items]        
Number of Interest rate derivatives held (contracts) | instrument 4      
Derivative cap interest rate (percent) 4.50%      
Scenario 2 [Member] | Interest Rate Cap | Secured Debt [Member] | FNMA        
Debt Instrument [Line Items]        
Number of Interest rate derivatives held (contracts) | instrument 1      
Derivative cap interest rate (percent) 5.00%      
[1] Amounts are adjusted to reflect interest rate swap and cap agreements in effect at December 31, 2015, and 2014, respectively, which results in the Company paying fixed interest payments over the terms of the interest rate swaps and on changes in interest rates above the strike rate of the cap.
[2] Includes capped balances