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Marketable Investment Securities - Schedule of Maturities of Debt Securities Classified as Cash Equivalents and Available-For-Sale Securities (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost $ 97.4 $ 102.4 $ 123.8 $ 68.5
Due within one year, Amortized cost 60.1      
Due after one year through five years, Amortized cost 51.7      
Due after five years, Amortized cost 0.0      
Total, Amortized cost 209.2 199.3    
Due within one year, Estimated fair value 59.9      
Due after one year through five years, Estimated fair value 51.3      
Due after five years, Estimated fair value 0.0      
Total, Estimated fair value 208.6 199.2    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost 91.1 83.5    
Cash equivalents, Estimated fair value 91.1 83.5    
Cash Equivalents [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost 6.3 18.9    
Cash equivalents, Estimated fair value $ 6.3 $ 18.9