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.