Marketable Investment Securities - Fair Value for Available-for-Sale Securities by Major Security Type and Class of Security (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	$ 165.2	$ 258.4
Cash and cash equivalents, Estimated fair value	165.2	258.4
Total:
Total, Amortized cost	340.9	398.9
Total, Gross unrealized holding gains	0.0	0.1
Total, Gross unrealized holding losses	(1.7)	(0.2)
Total, Estimated fair value	339.2	398.8
Cash
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	138.0	195.2
Cash and cash equivalents, Estimated fair value	138.0	195.2
Cash equivalents
Cash and cash equivalents:
Cash and cash equivalents, Amortized cost	27.2	63.2
Cash and cash equivalents, Estimated fair value	27.2	63.2
Corporate bonds and notes
Available-for-sale:
Available-for-sale, Amortized cost	109.1	105.7
Available-for-sale, Gross unrealized holding gains	0.0	0.1
Available-for-sale, Gross unrealized holding losses	(1.0)	(0.2)
Available-for-sale, Estimated fair value	108.1	105.6
Municipal bonds
Available-for-sale:
Available-for-sale, Amortized cost	21.6	16.1
Available-for-sale, Gross unrealized holding gains	0.0	0.0
Available-for-sale, Gross unrealized holding losses	(0.2)	0.0
Available-for-sale, Estimated fair value	21.4	16.1
Federal agency issues
Available-for-sale:
Available-for-sale, Amortized cost	18.0	6.8
Available-for-sale, Gross unrealized holding gains	0.0	0.0
Available-for-sale, Gross unrealized holding losses	(0.3)	0.0
Available-for-sale, Estimated fair value	17.7	6.8
US government securities
Available-for-sale:
Available-for-sale, Amortized cost	27.0	11.9
Available-for-sale, Gross unrealized holding gains	0.0	0.0
Available-for-sale, Gross unrealized holding losses	(0.2)	0.0
Available-for-sale, Estimated fair value	$ 26.8	$ 11.9