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Marketable Investment Securities - Schedule of Maturities of Debt Securities Classified as Cash Equivalents and Available-For-Sale Securities (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Jun. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | $ 93.3 | $ 110.9 | $ 87.9 | $ 102.4
Due within one year, Amortized cost | 75.9 |  |  |
Due after one year through five years, Amortized cost | 22.9 |  |  |
Due after five years, Amortized cost | 0.0 |  |  |
Total, Amortized cost | 192.1 | 211.9 |  |
Due within one year, Estimated fair value | 75.6 |  |  |
Due after one year through five years, Estimated fair value | 22.8 |  |  |
Due after five years, Estimated fair value | 0.0 |  |  |
Total, Estimated fair value | 191.7 | 211.3 |  |
Cash [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | 75.3 | 95.6 |  |
Cash equivalents, Estimated fair value | 75.3 | 95.6 |  |
Cash Equivalents [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | 18.0 | 15.3 |  |
Cash equivalents, Estimated fair value | $ 18.0 | $ 15.3 |  |

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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