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LONG-TERM DEBT - Subsidiary Revolving Credit Facility Due 2018 (Narrative) (Details) - USD ($)
3 Months Ended
Sep. 30, 2013
Dec. 31, 2017
Dec. 24, 2017
Nov. 30, 2017
Line of Credit Facility [Line Items]        
Maximum borrowing capacity   $ 550,000,000    
Revolving credit facility        
Line of Credit Facility [Line Items]        
Credit facility term 5 years      
Maximum borrowing capacity $ 75,000,000.0      
Outstanding under revolving credit facility   0 $ 405,000,000 $ 250,000,000
Letters of credit outstanding   $ 71,200,000