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Marketable Investment Securities - Fair Value for Available-for-Sale Securities by Major Security Type and Class of Security (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2019
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	$ 117.0	$ 163.7	$ 93.2
Cash and cash equivalents, Estimated fair value	117.0	163.7	93.2
Total:
Total, Amortized cost	170.9	253.5	191.2
Total, Gross unrealized holding gains	0.8	1.3	0.6
Total, Gross unrealized holding gains	0.0	0.0	0.0
Total, Estimated fair value	171.7	254.8	191.8
Cash
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	47.9	132.8	68.7
Cash and cash equivalents, Estimated fair value	47.9	132.8	68.7
Cash equivalents
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	69.1	30.9	24.5
Cash and cash equivalents, Estimated fair value	69.1	30.9	24.5
Corporate bonds and notes
Available-for-sale:
Available-for-sale, Amortized cost	28.8	50.1	64.0
Available-for-sale, Gross unrealized holding gains	0.5	0.8	0.6
Available-for-sale, Gross unrealized holding losses	0.0	0.0	0.0
Available-for-sale, Estimated fair value	29.3	50.9	64.6
Municipal bonds
Available-for-sale:
Available-for-sale, Amortized cost	9.4	17.8	15.3
Available-for-sale, Gross unrealized holding gains	0.2	0.2	0.0
Available-for-sale, Gross unrealized holding losses	0.0	0.0	0.0
Available-for-sale, Estimated fair value	9.6	18.0	15.3
Federal agency issues
Available-for-sale:
Available-for-sale, Amortized cost	4.0	5.5	9.0
Available-for-sale, Gross unrealized holding gains	0.0	0.1	0.0
Available-for-sale, Gross unrealized holding losses	0.0	0.0	0.0
Available-for-sale, Estimated fair value	4.0	5.6	9.0
US government securities
Available-for-sale:
Available-for-sale, Amortized cost	11.7	16.4	9.7
Available-for-sale, Gross unrealized holding gains	0.1	0.2	0.0
Available-for-sale, Gross unrealized holding losses	0.0	0.0	0.0
Available-for-sale, Estimated fair value	$ 11.8	$ 16.6	$ 9.7

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