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Marketable Investment Securities - Schedule of Maturities of Debt Securities Classified as Cash Equivalents and Available-For-Sale (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 110.9	$ 102.4	$ 68.5	$ 64.1
Due within one year, Amortized cost	70.0
Due after one year through five years, Amortized cost	31.0
Due after five years, Amortized cost	0.0
Total, Amortized cost	211.9	199.3
Due within one year, Estimated fair value	69.7
Due after one year through five years, Estimated fair value	30.7
Due after five years, Estimated fair value	0.0
Total, Estimated fair value	211.3	199.2
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	95.6	83.5
Cash equivalents, Estimated fair value	95.6	83.5
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	15.3	18.9
Cash equivalents, Estimated fair value	$ 15.3	$ 18.9

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