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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	Dec. 31, 2019	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 81.2	$ 93.2
Available-for-sale, Amortized cost	188.6	191.2
Gross unrealized holding gains	0.6	0.6
Available-for-sale, Estimated fair value	189.2	191.8
Amortized cost	188.6	191.2
Estimated fair value	189.2	191.8
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	63.2	68.7
Cash and cash equivalents, Estimated fair value	63.2	68.7
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	18.0	24.5
Cash and cash equivalents, Estimated fair value	18.0	24.5
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	81.2	93.2
Cash and cash equivalents, Estimated fair value	81.2	93.2
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	67.2	64.0
Gross unrealized holding gains	0.4	0.6
Available-for-sale, Estimated fair value	67.6	64.6
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	20.8	15.3
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	20.9	15.3
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	4.0	9.0
Available-for-sale, Estimated fair value	4.0	9.0
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	15.4	9.7
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	$ 15.5	$ 9.7

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