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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, 2019 | Jun. 30, 2018 | Jun. 30, 2017 | [1] | Jun. 30, 2016 | [1]
--- | --- | --- | --- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | $ 93.2 | $ 110.9 | [1] | $ 102.4 | $ 68.5
Available-for-sale, Amortized cost | 191.2 | 211.9 |  |  |
Gross unrealized holding gains | 0.6 |  |  |  |
Gross unrealized holding losses |  | (0.6) |  |  |
Available-for-sale, Estimated fair value | 191.8 | 211.3 |  |  |
Amortized cost | 191.2 | 211.9 |  |  |
Estimated fair value | 191.8 | 211.3 |  |  |
Cash [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 68.7 | 95.6 |  |  |
Cash and cash equivalents, Estimated fair value | 68.7 | 95.6 |  |  |
Cash Equivalents [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 24.5 | 15.3 |  |  |
Cash and cash equivalents, Estimated fair value | 24.5 | 15.3 |  |  |
Total Cash and Cash Equivalents [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 93.2 | 110.9 |  |  |
Cash and cash equivalents, Estimated fair value | 93.2 | 110.9 |  |  |
Corporate Bonds and Notes [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 64.0 | 50.8 |  |  |
Gross unrealized holding gains | 0.6 |  |  |  |
Gross unrealized holding losses |  | (0.3) |  |  |
Available-for-sale, Estimated fair value | 64.6 | 50.5 |  |  |
Municipal Bonds [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 15.3 | 29.3 |  |  |
Gross unrealized holding losses |  | (0.1) |  |  |
Available-for-sale, Estimated fair value | 15.3 | 29.2 |  |  |
Federal Agency Issues [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 9.0 | 12.6 |  |  |
Gross unrealized holding losses |  | (0.1) |  |  |
Available-for-sale, Estimated fair value | 9.0 | 12.5 |  |  |
US Government Securities [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 9.7 | 8.3 |  |  |
Gross unrealized holding losses |  | (0.1) |  |  |
Available-for-sale, Estimated fair value | $ 9.7 | $ 8.2 |  |  |
[1] | Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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