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Marketable Investment Securities - Schedule of Fair Value Debt Securities Classified as Available-for-Sale Securities by Major Security Type and Class of Security (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
Available-for-sale, Amortized cost	253.5	191.2
Gross unrealized holding gains	1.3	0.6
Available-for-sale, Estimated fair value	254.8	191.8
Amortized cost	253.5	191.2
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 and 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 and cash equivalents, Estimated fair value	30.9	24.5
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	163.7	93.2
Cash and cash equivalents, Estimated fair value	163.7	93.2
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	50.1	64.0
Gross unrealized holding gains	0.8	0.6
Available-for-sale, Estimated fair value	50.9	64.6
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	17.8	15.3
Gross unrealized holding gains	0.2
Available-for-sale, Estimated fair value	18.0	15.3
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	5.5	9.0
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	5.6	9.0
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	16.4	9.7
Gross unrealized holding gains	0.2
Available-for-sale, Estimated fair value	$ 16.6	$ 9.7

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