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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, 2019	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 89.9	$ 93.2
Due within one year, Amortized cost	54.9
Due after one year through five years, Amortized cost	48.7
Due after five years, Amortized cost	0.0
Total, Amortized cost	193.5	191.2
Due within one year, Estimated fair value	55.0
Due after one year through five years, Estimated fair value	49.2
Due after five years, Estimated fair value	0.0
Total, Estimated fair value	194.1	191.8
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	67.2	68.7
Cash equivalents, Estimated fair value	67.2	68.7
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	22.7	24.5
Cash equivalents, Estimated fair value	$ 22.7	$ 24.5

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