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Debt - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2016
Mar. 31, 2015
Jan. 07, 2016
Jul. 22, 2015
Debt            
Total principal $ 385,886,000 $ 488,926,000 $ 385,886,000      
Interest expense, debt     $ 3,800,000 $ 1,400,000    
Credit agreement            
Debt            
Term of extension option     1 year      
Unused fee (as a percent)     0.25%      
Amount outstanding 121,000,000   $ 121,000,000      
Current borrowing capacity 294,600,000.0   294,600,000.0      
Remaining borrowing capacity $ 334,400,000   $ 334,400,000      
Credit agreement | Operating Partnership            
Debt            
Interest rate at end of period (as a percent) 3.76%   3.76%      
Senior notes            
Debt            
Debt instrument, face amount         $ 150,000,000  
Credit agreement amendment | Credit agreement            
Debt            
Maximum borrowing capacity as a percentage of maximum principal amount           10.00%
Increase in maximum borrowing capacity           $ 350,000,000
Maximum borrowing capacity under accordion feature           $ 1,100,000,000.0
Series A            
Debt            
Interest rate (as a percent)         4.03%  
Series A | Senior notes            
Debt            
Debt instrument, face amount         $ 15,000,000  
Series B            
Debt            
Interest rate (as a percent)         4.43%  
Series B | Senior notes            
Debt            
Debt instrument, face amount         $ 45,000,000  
Series C            
Debt            
Interest rate (as a percent)         4.57%  
Series C | Senior notes            
Debt            
Debt instrument, face amount         $ 45,000,000  
Series D            
Debt            
Interest rate (as a percent)         4.74%  
Series D | Senior notes            
Debt            
Debt instrument, face amount         $ 45,000,000  
Base rate | Credit rating at least A- or A3 | Base rate loans            
Debt            
Reference rate (as a percent)     0.00%      
Base rate | Credit rating at least BBBplus or BAA1 | Base rate loans            
Debt            
Reference rate (as a percent)     0.00%      
Base rate | Credit rating at least BBB or BAA2 | Base rate loans            
Debt            
Reference rate (as a percent)     0.10%      
Base rate | Credit rating at least BBB- or BAA3 | Base rate loans            
Debt            
Reference rate (as a percent)     0.20%      
Base rate | Credit rating below BBB- or BAA3 | Base rate loans            
Debt            
Reference rate (as a percent)     0.60%      
London Interbank Offered Rate (LIBOR) | Credit agreement            
Debt            
Reference rate (as a percent) 1.20% 1.20% 1.20%      
London Interbank Offered Rate (LIBOR) | Credit rating at least A- or A3 | Adjusted LIBOR rate loans and letter of credit            
Debt            
Reference rate (as a percent) 0.85%          
London Interbank Offered Rate (LIBOR) | Credit rating at least BBBplus or BAA1 | Adjusted LIBOR rate loans and letter of credit            
Debt            
Reference rate (as a percent) 0.90%          
London Interbank Offered Rate (LIBOR) | Credit rating at least BBB or BAA2 | Adjusted LIBOR rate loans and letter of credit            
Debt            
Reference rate (as a percent) 1.00%          
London Interbank Offered Rate (LIBOR) | Credit rating at least BBB- or BAA3 | Adjusted LIBOR rate loans and letter of credit            
Debt            
Reference rate (as a percent) 1.20%          
London Interbank Offered Rate (LIBOR) | Credit rating below BBB- or BAA3 | Adjusted LIBOR rate loans and letter of credit            
Debt            
Reference rate (as a percent) 1.55%