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Senior Notes (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Sep. 30, 2019
Jun. 30, 2019
Dec. 31, 2018
May 02, 2013
Nov. 02, 2010
Senior Notes [Line Items]          
Carrying value of senior notes   $ 542,740 $ 542,740    
Unamortized Debt Issuance Expense   1,400 2,600    
Senior note redemption price, percent 100.00%        
Senior Notes Due 2020 [Member]          
Senior Notes [Line Items]          
Initial borrowing in senior notes         $ 300,000
Portion of proceeds from senior notes used to repay outstanding term debt   200,000      
Carrying value of senior notes   292,740 292,740    
Unamortized Debt Issuance Expense   800 1,200    
Senior Notes Due 2021 [Member]          
Senior Notes [Line Items]          
Initial borrowing in senior notes       $ 250,000  
Portion of proceeds from senior notes used to repay outstanding term debt   200,000      
Carrying value of senior notes   250,000 250,000    
Unamortized Debt Issuance Expense   $ 1,200 $ 1,500