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Fair Value - Financial Assets Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Millions
Dec. 31, 2017
Jun. 30, 2017
Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total available-for-sale securities $ 219.7 $ 361.7
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total available-for-sale securities 1.5 1.5
Money Market Funds [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total available-for-sale securities 219.7 361.7
Auction Rate Securities [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total available-for-sale securities $ 1.5 $ 1.5