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Marketable Investment Securities - Schedule of Fair Value for Available-for-Sale Securities by Major Security Type and Class of Security (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2017	[1]	Jun. 30, 2016	[1]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 93.2	$ 110.9	[1]	$ 102.4		$ 68.5
Available-for-sale, Amortized cost	191.2	211.9
Gross unrealized holding gains	0.6
Gross unrealized holding losses		(0.6)
Available-for-sale, Estimated fair value	191.8	211.3
Amortized cost	191.2	211.9
Estimated fair value	191.8	211.3
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	68.7	95.6
Cash and cash equivalents, Estimated fair value	68.7	95.6
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	24.5	15.3
Cash and cash equivalents, Estimated fair value	24.5	15.3
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	93.2	110.9
Cash and cash equivalents, Estimated fair value	93.2	110.9
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	64.0	50.8
Gross unrealized holding gains	0.6
Gross unrealized holding losses		(0.3)
Available-for-sale, Estimated fair value	64.6	50.5
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	15.3	29.3
Gross unrealized holding losses		(0.1)
Available-for-sale, Estimated fair value	15.3	29.2
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	9.0	12.6
Gross unrealized holding losses		(0.1)
Available-for-sale, Estimated fair value	9.0	12.5
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	9.7	8.3
Gross unrealized holding losses		(0.1)
Available-for-sale, Estimated fair value	$ 9.7	$ 8.2
[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.

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