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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	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 93.3	$ 110.9	$ 87.9	$ 102.4
Due within one year, Amortized cost	75.9
Due after one year through five years, Amortized cost	22.9
Due after five years, Amortized cost	0.0
Total, Amortized cost	192.1	211.9
Due within one year, Estimated fair value	75.6
Due after one year through five years, Estimated fair value	22.8
Due after five years, Estimated fair value	0.0
Total, Estimated fair value	191.7	211.3
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	75.3	95.6
Cash equivalents, Estimated fair value	75.3	95.6
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	18.0	15.3
Cash equivalents, Estimated fair value	$ 18.0	$ 15.3

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