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Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | $ 3,550.4 | $ 4,234.5
Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total marketable securities | 761.3 | 618.8
Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total marketable securities | 3,566.7 | 4,234.5
Unrestricted | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 41.3 | 61.6
Unrestricted | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 0.0 | 0.0
Restricted | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 720.0 | 557.2
Restricted | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 16.3 | 0.0
Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total marketable securities | 4,328.0 | 4,853.3
Estimate of Fair Value Measurement | Unrestricted | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 41.3 | 61.6
Estimate of Fair Value Measurement | Restricted | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 736.3 | 557.2
Corporate bonds | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 0.0 | 0.0
Corporate bonds | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 2,836.0 | 3,988.1
Corporate bonds | Unrestricted |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 2,836.0 | 3,988.1
Corporate bonds | Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 2,836.0 | 3,988.1
U.S. government and government agency obligations | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 0.0 | 0.0
U.S. government and government agency obligations | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 134.7 | 54.4
U.S. government and government agency obligations | Unrestricted |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 134.7 | 54.4
U.S. government and government agency obligations | Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 134.7 | 54.4
Sovereign bonds | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 0.0 | 0.0
Sovereign bonds | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 66.6 | 27.3
Sovereign bonds | Unrestricted |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 66.6 | 27.3
Sovereign bonds | Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 66.6 | 27.3
Commercial paper | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 0.0 | 0.0
Commercial paper | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 390.6 | 92.3
Commercial paper | Unrestricted |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 390.6 | 92.3
Commercial paper | Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 390.6 | 92.3
Certificates of deposit | Measured on a recurring basis | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 0.0 | 0.0
Certificates of deposit | Measured on a recurring basis | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 122.5 | 72.4
Certificates of deposit | Unrestricted |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | 122.5 | 72.4
Certificates of deposit | Estimate of Fair Value Measurement | Measured on a recurring basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale debt securities: | $ 122.5 | $ 72.4

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