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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	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 97.4	$ 102.4	$ 123.8	$ 68.5
Available-for-sale, Amortized cost	209.2	199.3
Gross unrealized holding gains		0.1
Gross unrealized holding losses	(0.6)	(0.2)
Available-for-sale, Estimated fair value	208.6	199.2
Amortized cost	209.2	199.3
Estimated fair value	208.6	199.2
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	91.1	83.5
Cash and cash equivalents, Estimated fair value	91.1	83.5
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	6.3	18.9
Cash and cash equivalents, Estimated fair value	6.3	18.9
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	97.4	102.4
Cash and cash equivalents, Estimated fair value	97.4	102.4
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	59.9	45.4
Gross unrealized holding gains		0.1
Gross unrealized holding losses	(0.3)	(0.1)
Available-for-sale, Estimated fair value	59.6	45.4
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	31.0	32.7
Gross unrealized holding losses	(0.1)
Available-for-sale, Estimated fair value	30.9	32.7
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	12.6	11.6
Gross unrealized holding losses	(0.1)	(0.1)
Available-for-sale, Estimated fair value	12.5	11.5
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	8.3	7.2
Gross unrealized holding losses	(0.1)
Available-for-sale, Estimated fair value	$ 8.2	$ 7.2

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Investments and cash at fair value.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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