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Debt - Schedule of Contractual Interest Expense and Amortization of Discount on Notes (Detail) - USD ($)
$ in Millions
6 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Amortization of discount and debt issuance costs $ 8.7 $ 8.4
0.50% Convertible Senior Notes due 2022 [Member]    
Debt Instrument [Line Items]    
Interest expense 1.3  
Amortization of discount and debt issuance costs 8.7  
Total interest $ 10.0