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Debt - Schedule of Contractual Interest Expense and Amortization of Discount on Notes (Detail) - USD ($)
$ in Millions
3 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]    
Amortization of discount and debt issuance costs $ 4.4 $ 4.3
0.50% Convertible Senior Notes due 2022 [Member]    
Debt Instrument [Line Items]    
Interest expense 0.7  
Amortization of discount and debt issuance costs 4.4  
Total interest $ 5.1