Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Available-for-sale debt securities	$ 3,689.1	$ 4,234.5
Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Total marketable securities	839.8	618.8
Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Total marketable securities	3,689.1	4,234.5
Unrestricted \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Equity securities	48.3	61.6
Unrestricted \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Equity securities	0.0	0.0
Restricted \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Equity securities	791.5	557.2
Restricted \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Equity securities	0.0	0.0
Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Total marketable securities	4,528.9	4,853.3
Estimate of Fair Value Measurement \| Unrestricted \| Measured on a recurring basis
Available-for-sale debt securities:
Equity securities	48.3	61.6
Estimate of Fair Value Measurement \| Restricted \| Measured on a recurring basis
Available-for-sale debt securities:
Equity securities	791.5	557.2
Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,090.3	3,988.1
Corporate bonds \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Corporate bonds \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Available-for-sale debt securities	3,090.3	3,988.1
Corporate bonds \| Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	3,090.3	3,988.1
U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	128.9	54.4
U.S. government and government agency obligations \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
U.S. government and government agency obligations \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Available-for-sale debt securities	128.9	54.4
U.S. government and government agency obligations \| Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	128.9	54.4
Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	66.3	27.3
Sovereign bonds \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Sovereign bonds \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Available-for-sale debt securities	66.3	27.3
Sovereign bonds \| Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	66.3	27.3
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	276.1	92.3
Commercial paper \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Commercial paper \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Available-for-sale debt securities	276.1	92.3
Commercial paper \| Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	276.1	92.3
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	127.5	72.4
Certificates of deposit \| Measured on a recurring basis \| Level 1
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Certificates of deposit \| Measured on a recurring basis \| Level 2
Available-for-sale debt securities:
Available-for-sale debt securities	127.5	72.4
Certificates of deposit \| Estimate of Fair Value Measurement \| Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	$ 127.5	$ 72.4

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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