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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, 2017	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 102.4	$ 68.5	$ 64.1	$ 64.8
Available-for-sale, Amortized cost	199.3	238.4
Gross unrealized holding gains	0.1	0.5
Gross unrealized holding losses	(0.2)
Available-for-sale, Estimated fair value	199.2	238.9
Amortized cost	199.3	238.4
Estimated fair value	199.2	238.9
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	83.5	66.1
Cash and cash equivalents, Estimated fair value	83.5	66.1
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	18.9	2.4
Cash and cash equivalents, Estimated fair value	18.9	2.4
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	102.4	68.5
Cash and cash equivalents, Estimated fair value	102.4	68.5
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	45.4	50.8
Gross unrealized holding gains	0.1	0.2
Gross unrealized holding losses	(0.1)
Available-for-sale, Estimated fair value	45.4	51.0
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	32.7	85.4
Gross unrealized holding gains		0.2
Available-for-sale, Estimated fair value	32.7	85.6
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	11.6	25.5
Gross unrealized holding losses	(0.1)
Available-for-sale, Estimated fair value	11.5	25.5
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	7.2	8.2
Gross unrealized holding gains		0.1
Available-for-sale, Estimated fair value	$ 7.2	$ 8.3

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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Sum of investments and unrestricted cash as of the balance sheet date.

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