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Debt - 2014 Senior Notes (Details) - USD ($)
12 Months Ended
Mar. 16, 2018
Oct. 06, 2014
Sep. 30, 2018
The 2022 Notes      
Debt      
Redemption price equal to principal amount (in percent) 104.313%    
Prepayment premium redemption     $ 34,500,000
2014 Senior Notes | The 2022 Notes      
Debt      
Principal amount   $ 800,000,000  
Interest rate (as a percent)   5.75%  
2014 Senior Notes | The 2024 Notes      
Debt      
Principal amount   $ 800,000,000  
Interest rate (as a percent)   5.875%  
Fair value of debt instrument     $ 844,000,000
2014 Senior Notes | The 2024 Notes | Prior to July 15 2024      
Debt      
Redemption price equal to principal amount (in percent)   100.00%  
2014 Senior Notes | The 2024 Notes | On or after July 15, 2024      
Debt      
Redemption price equal to principal amount (in percent)   100.00%