Marketable Investment Securities - Schedule of Fair Value for Available-for-Sale Securities by Major Security Type and Class of Security (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
Available-for-sale, Amortized cost	223.8	191.2
Gross unrealized holding gains	0.7	0.6
Gross unrealized holding losses	(0.2)
Available-for-sale, Estimated fair value	224.3	191.8
Amortized cost	223.8	191.2
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 and 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 and cash equivalents, Estimated fair value	17.3	24.5
Total Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents, Amortized cost	121.0	93.2
Cash and cash equivalents, Estimated fair value	121.0	93.2
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	61.0	64.0
Gross unrealized holding gains	0.2	0.6
Gross unrealized holding losses	(0.2)
Available-for-sale, Estimated fair value	61.0	64.6
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	19.9	15.3
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	20.0	15.3
Federal Agency Issues [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	5.5	9.0
Gross unrealized holding gains	0.1
Available-for-sale, Estimated fair value	5.6	9.0
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	16.4	9.7
Gross unrealized holding gains	0.3
Available-for-sale, Estimated fair value	$ 16.7	$ 9.7