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LONG-TERM DEBT - 6.75 Percent Notes (Details) - USD ($)
12 Months Ended
May 31, 2013
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Debt Instrument [Line Items]        
Proceeds from debt issuance   $ 325,000,000 $ 0 $ 225,000,000
8.125% notes due 2015        
Debt Instrument [Line Items]        
Stated interest rate   8.125%    
Senior Notes | 6.75% notes due 2021        
Debt Instrument [Line Items]        
Stated interest rate   6.75%    
Debt instrument, face amount $ 275,000,000      
Debt term 8 years      
Issuance price of principle 100.00%      
Proceeds from debt issuance $ 275,000,000      
Redemption price percentage   100.00%    
Percentage of principal that may be redeemed   35.00%    
Percentage of principal outstanding   65.00%    
Debt outstanding, gross   $ 175,000,000    
Senior Notes | 6.75% notes due 2021 | Redemption period one        
Debt Instrument [Line Items]        
Redemption price percentage   105.063%    
Senior Notes | 6.75% notes due 2021 | Redemption period two        
Debt Instrument [Line Items]        
Redemption price percentage   103.375%    
Senior Notes | 6.75% notes due 2021 | Redemption period three        
Debt Instrument [Line Items]        
Redemption price percentage   101.688%    
Senior Notes | 6.75% notes due 2021 | Redemption period four        
Debt Instrument [Line Items]        
Redemption price percentage   100.00%    
Senior Notes | 6.75% notes due 2021 | Equity clawback        
Debt Instrument [Line Items]        
Redemption price percentage   106.75%    
Senior Notes | 6.75% notes due 2021 | Change in control        
Debt Instrument [Line Items]        
Redemption price percentage   101.00%    
Senior Notes | 8.125% notes due 2015        
Debt Instrument [Line Items]        
Repurchased principal amount of debt $ 167,000,000