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Fair Value Measurements - Narrative (Details) - USD ($)
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Aug. 31, 2017
Jul. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Length of time to be considered time deposit 3 months      
Expiration period 12 months      
Senior Notes | 2024 Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Interest rate of debt       5.50%
Aggregate principal amount       $ 375,000,000
Senior Notes | 2025 Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Interest rate of debt     4.375%  
Aggregate principal amount     $ 400,000,000  
Foreign Exchange Contract | Not Designated as Hedging Instrument        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Outstanding derivative contracts notional value $ 74,000,000 $ 47,000,000