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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, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 87.9	$ 102.4	$ 86.9	$ 68.5
Available-for-sale, Amortized cost	198.4	199.3
Gross unrealized holding gains	0.1	0.1
Gross unrealized holding losses	(0.1)	(0.2)
Available-for-sale, Estimated fair value	198.4	199.2
Amortized cost	198.4	199.3
Estimated fair value	198.4	199.2
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	82.3	83.5
Cash and cash equivalents, Estimated fair value	82.3	83.5
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	5.6	18.9
Cash and cash equivalents, Estimated fair value	5.6	18.9
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	87.9	102.4
Cash and cash equivalents, Estimated fair value	87.9	102.4
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	62.2	45.4
Gross unrealized holding gains	0.1	0.1
Gross unrealized holding losses	(0.1)	(0.1)
Available-for-sale, Estimated fair value	62.2	45.4
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	31.2	32.7
Available-for-sale, Estimated fair value	31.2	32.7
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	9.5	11.6
Gross unrealized holding losses		(0.1)
Available-for-sale, Estimated fair value	9.5	11.5
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	7.6	7.2
Available-for-sale, Estimated fair value	$ 7.6	$ 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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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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