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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
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost $ 88.7 $ 102.4 $ 108.0 $ 68.5
Due within one year, Amortized cost 54.9      
Due after one year through five years, Amortized cost 58.8      
Due after five years, Amortized cost 0.0      
Total, Amortized cost 202.4 199.3    
Due within one year, Estimated fair value 54.8      
Due after one year through five years, Estimated fair value 58.5      
Due after five years, Estimated fair value 0.0      
Total, Estimated fair value 202.0 199.2    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost 85.9 83.5    
Cash equivalents, Estimated fair value 85.9 83.5    
Cash Equivalents [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents, Amortized cost 2.8 18.9    
Cash equivalents, Estimated fair value $ 2.8 $ 18.9