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Subsequent Events - Debt Obligations (Details) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Debt Obligations        
Proceeds from issuance of senior unsecured notes   $ 77,500,000 $ 200,000,000 $ 30,000,000
Outstanding Balance   609,391,000 571,872,000  
Bank Borrowings - Line of Credit        
Debt Obligations        
Maximum available under facility   600,000,000    
Outstanding Balance   107,100,000 120,500,000  
Available for borrowing   492,900,000 479,500,000  
Bank Borrowings - Line of Credit | Subsequent Event        
Debt Obligations        
Outstanding Balance $ 0      
Available for borrowing 600,000,000      
Senior Unsecured Notes        
Debt Obligations        
Outstanding Balance   502,291,000 451,372,000  
Available for borrowing   $ 22,500,000 33,333,000  
Private Shelf Agreement Prudential | Senior Unsecured Notes        
Debt Obligations        
Repayments of debt     29,167,000  
Debt instrument term   10 years    
Face amount of debt   $ 37,500,000 $ 100,000,000  
Fixed interest rate (as a percent)   4.15% 4.50%  
Available for borrowing   $ 22,500,000    
Private Shelf Agreement Prudential | Senior Unsecured Notes | Subsequent Event        
Debt Obligations        
Repayments of debt 4,167,000      
Maximum available under facility $ 337,500,000      
Debt instrument term 15 years      
Face amount of debt $ 100,000,000      
Fixed interest rate (as a percent) 4.50%      
Outstanding Balance $ 598,124,000      
Available for borrowing $ 36,667,000