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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	Sep. 30, 2019	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 89.9	$ 93.2
Available-for-sale, Amortized cost	193.5	191.2
Gross unrealized holding gains	0.6	0.6
Available-for-sale, Estimated fair value	194.1	191.8
Amortized cost	193.5	191.2
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 and 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 and cash equivalents, Estimated fair value	22.7	24.5
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	89.9	93.2
Cash and cash equivalents, Estimated fair value	89.9	93.2
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	68.7	64.0
Gross unrealized holding gains	0.4	0.6
Available-for-sale, Estimated fair value	69.1	64.6
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	15.0	15.3
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	15.1	15.3
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	7.0	9.0
Available-for-sale, Estimated fair value	7.0	9.0
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	12.9	9.7
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	$ 13.0	$ 9.7

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