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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, 2020	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 163.7	$ 93.2
Due within one year, Amortized cost	53.7
Due after one year through five years, Amortized cost	36.1
Total, Amortized cost	253.5	191.2
Due within one year, Estimated fair value	54.1
Due after one year through five years, Estimated fair value	37.0
Total, Estimated fair value	254.8	191.8
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	132.8	68.7
Cash equivalents, Estimated fair value	132.8	68.7
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	30.9	24.5
Cash equivalents, Estimated fair value	$ 30.9	$ 24.5

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Investments and cash at fair value.

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Sum of investments and unrestricted cash as of the balance sheet date.

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