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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, 2021
Dec. 31, 2020
Cash and cash equivalents:    
Cash and cash equivalents, Amortized cost $ 258.4 $ 117.0
Cash and cash equivalents, Estimated fair value 258.4 117.0
Total:    
Total, Amortized cost 398.9 170.9
Total, Gross unrealized holding gains 0.1 0.8
Total, Gross unrealized holding gains (0.2) 0.0
Total, Estimated fair value 398.8 171.7
Cash    
Cash and cash equivalents:    
Cash and cash equivalents, Amortized cost 195.2 47.9
Cash and cash equivalents, Estimated fair value 195.2 47.9
Cash equivalents    
Cash and cash equivalents:    
Cash and cash equivalents, Amortized cost 63.2 69.1
Cash and cash equivalents, Estimated fair value 63.2 69.1
Corporate bonds and notes    
Available-for-sale:    
Available-for-sale, Amortized cost 105.7 28.8
Available-for-sale, Gross unrealized holding gains 0.1 0.5
Available-for-sale, Gross unrealized holding losses (0.2) 0.0
Available-for-sale, Estimated fair value 105.6 29.3
Municipal bonds    
Available-for-sale:    
Available-for-sale, Amortized cost 16.1 9.4
Available-for-sale, Gross unrealized holding gains 0.0 0.2
Available-for-sale, Gross unrealized holding losses 0.0 0.0
Available-for-sale, Estimated fair value 16.1 9.6
Federal agency issues    
Available-for-sale:    
Available-for-sale, Amortized cost 6.8 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 6.8 4.0
US government securities    
Available-for-sale:    
Available-for-sale, Amortized cost 11.9 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 $ 11.9 $ 11.8