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Convertible Senior Notes - Convertible Senior Notes Due 2017 (Narrative) (Details) - USD ($)
1 Months Ended 9 Months Ended
Jun. 01, 2017
Jun. 30, 2016
Jun. 30, 2012
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]          
Repayments of convertible senior notes       $ 0 $ 55,000,000
Senior Notes | Convertible Senior Notes Due 2017          
Debt Instrument [Line Items]          
Principal     $ 275,000,000    
Interest rate     1.375%    
Proceeds from offering     $ 266,200,000    
Repayments of convertible senior notes $ 55,400,000 $ 323,200,000      
Aggregate principal amount repurchased   220,000,000      
Net cash receipt   $ 12,600,000      
Debt conversion, converted instrument, shares issued (in shares) 819,901