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Debt Obligations - Summary (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Obligations        
Applicable Interest Rate (as a percent)   4.27%    
Outstanding Balance   $ 667,502 $ 609,391  
Available for borrowing   567,167 515,400  
Borrowings   213,000 201,100 $ 491,000
Payments on debt   $ 154,767 163,667 199,667
Bank Borrowings        
Debt Obligations        
Applicable Interest Rate (as a percent)   2.90%    
Outstanding Balance   $ 96,500 107,100  
Available for borrowing   503,500 492,900  
Borrowings   113,000 123,600 291,000
Payments on debt   $ 123,600 137,000 170,500
Bank Borrowings | Subsequent Event        
Debt Obligations        
Outstanding Balance $ 120,500      
Available for borrowing 479,500      
Borrowings 24,000      
Senior Unsecured Notes        
Debt Obligations        
Applicable Interest Rate (as a percent)   4.50%    
Outstanding Balance   $ 571,002 502,291  
Available for borrowing   63,667 22,500  
Borrowings   100,000 77,500 200,000
Payments on debt   $ 31,167 $ 26,667 $ 29,167
Senior Unsecured Notes | Subsequent Event        
Debt Obligations        
Outstanding Balance 566,835      
Available for borrowing 67,833      
Payments on debt $ 4,167