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Long-Term Debt - Revolving Credit Facility (Details) - Revolving Credit Facility
3 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Line of Credit Facility [Line Items]      
Borrowings outstanding $ 0 $ 0  
Letters of credit outstanding $ 0 $ 0  
LIBOR      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 3.00%    
Unused capacity, commitment fee percentage 0.45%    
Triggering Event One      
Line of Credit Facility [Line Items]      
Debt-to-EBITDA ratio 2.25    
Amended Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity     $ 525,000,000
Deferred issuance cost     $ 4,000,000
Collateral test, maximum ratio 1.0    
Value of company assets collateralized $ 726,000,000    
Maximum limit on issuance, letters of credit $ 100,000,000    
Overnight Revolving Credit Loans | Prime Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 2.00%