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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, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | [1] | Jun. 30, 2017 | [1]
--- | --- | --- | --- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | $ 90.6 | $ 110.9 | [1] | $ 88.7 | $ 102.4
Available-for-sale, Amortized cost | 195.6 | 211.9 |  |  |
Gross unrealized holding losses | (0.3) | (0.6) |  |  |
Available-for-sale, Estimated fair value | 195.3 | 211.3 |  |  |
Amortized cost | 195.6 | 211.9 |  |  |
Estimated fair value | 195.3 | 211.3 |  |  |
Cash [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 74.2 | 95.6 |  |  |
Cash and cash equivalents, Estimated fair value | 74.2 | 95.6 |  |  |
Cash Equivalents [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 16.4 | 15.3 |  |  |
Cash and cash equivalents, Estimated fair value | 16.4 | 15.3 |  |  |
Total Cash and Cash Equivalents [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Cash and cash equivalents, Amortized cost | 90.6 | 110.9 |  |  |
Cash and cash equivalents, Estimated fair value | 90.6 | 110.9 |  |  |
Corporate Bonds and Notes [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 68.6 | 50.8 |  |  |
Gross unrealized holding losses | (0.2) | (0.3) |  |  |
Available-for-sale, Estimated fair value | 68.4 | 50.5 |  |  |
Municipal Bonds [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 17.0 | 29.3 |  |  |
Gross unrealized holding losses |  | (0.1) |  |  |
Available-for-sale, Estimated fair value | 17.0 | 29.2 |  |  |
Federal Agency Issues [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 12.1 | 12.6 |  |  |
Gross unrealized holding losses | (0.1) | (0.1) |  |  |
Available-for-sale, Estimated fair value | 12.0 | 12.5 |  |  |
US Government Securities [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available-for-sale, Amortized cost | 7.3 | 8.3 |  |  |
Gross unrealized holding losses |  | (0.1) |  |  |
Available-for-sale, Estimated fair value | $ 7.3 | $ 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 contained in Part I, Item 1 of this report for additional information.

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