Marketable Investment Securities - Schedule of Maturities of Debt Securities Classified as Cash Equivalents and Available-For-Sale Securities (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
Due within one year, Amortized cost	75.1
Due after one year through five years, Amortized cost	29.9
Due after five years, Amortized cost	0.0
Total, Amortized cost	195.6	211.9
Due within one year, Estimated fair value	74.8
Due after one year through five years, Estimated fair value	29.9
Due after five years, Estimated fair value	0.0
Total, 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 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 equivalents, Estimated fair value	$ 16.4	$ 15.3

[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.