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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	Mar. 31, 2021	Dec. 31, 2020
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	$ 148.9	$ 117.0
Cash and cash equivalents, Estimated fair value	148.9	117.0
Total:
Total, Amortized cost	187.5	170.9
Total, Gross unrealized holding gains	0.5	0.8
Total, Gross unrealized holding losses	0.0	0.0
Total, Estimated fair value	188.0	171.7
Cash
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	65.9	47.9
Cash and cash equivalents, Estimated fair value	65.9	47.9
Cash equivalents
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	83.0	69.1
Cash and cash equivalents, Estimated fair value	83.0	69.1
Corporate bonds and notes
Available-for-sale:
Available-for-sale, Amortized cost	22.7	28.8
Available-for-sale, Gross unrealized holding gains	0.4	0.5
Available-for-sale, Gross unrealized holding losses	0.0	0.0
Available-for-sale, Estimated fair value	23.1	29.3
Municipal bonds
Available-for-sale:
Available-for-sale, Amortized cost	8.2	9.4
Available-for-sale, Gross unrealized holding gains	0.1	0.2
Available-for-sale, Gross unrealized holding losses	0.0	0.0
Available-for-sale, Estimated fair value	8.3	9.6
Federal agency issues
Available-for-sale:
Available-for-sale, Amortized cost	4.0	4.0
Available-for-sale, Gross unrealized holding gains	0.0	0.0
Available-for-sale, Gross unrealized holding losses	0.0	0.0
Available-for-sale, Estimated fair value	4.0	4.0
US government securities
Available-for-sale:
Available-for-sale, Amortized cost	3.7	11.7
Available-for-sale, Gross unrealized holding gains	0.0	0.1
Available-for-sale, Gross unrealized holding losses	0.0	0.0
Available-for-sale, Estimated fair value	$ 3.7	$ 11.8

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