Marketable Investment Securities - Schedule of Maturities of Debt Securities Classified as Cash Equivalents and Available-For-Sale Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	$ 121.0	$ 93.2
Due within one year, Amortized cost	60.2
Due after one year through five years, Amortized cost	42.6
Total, Amortized cost	223.8	191.2
Due within one year, Estimated fair value	60.5
Due after one year through five years, Estimated fair value	42.8
Total, Estimated fair value	224.3	191.8
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	103.7	68.7
Cash equivalents, Estimated fair value	103.7	68.7
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	17.3	24.5
Cash equivalents, Estimated fair value	$ 17.3	$ 24.5