Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale debt securities:
Available-for-sale debt securities	$ 4,607.2	$ 3,689.1
Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,632.2	3,090.3
U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	487.8	128.9
Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.7	66.3
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	254.3	276.1
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	167.2	127.5
Measured on a recurring basis | Level 1
Available-for-sale debt securities:
Total marketable securities	1,002.4	839.8
Measured on a recurring basis | Level 1 | Unrestricted
Available-for-sale debt securities:
Equity securities	77.7	48.3
Measured on a recurring basis | Level 1 | Restricted
Available-for-sale debt securities:
Equity securities	924.7	791.5
Measured on a recurring basis | Level 1 | Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Measured on a recurring basis | Level 1 | U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Measured on a recurring basis | Level 1 | Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Measured on a recurring basis | Level 1 | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Measured on a recurring basis | Level 1 | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Measured on a recurring basis | Level 2
Available-for-sale debt securities:
Total marketable securities	4,607.2	3,689.1
Measured on a recurring basis | Level 2 | Unrestricted
Available-for-sale debt securities:
Equity securities	0.0	0.0
Measured on a recurring basis | Level 2 | Restricted
Available-for-sale debt securities:
Equity securities	0.0	0.0
Measured on a recurring basis | Level 2 | Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,632.2	3,090.3
Measured on a recurring basis | Level 2 | U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	487.8	128.9
Measured on a recurring basis | Level 2 | Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.7	66.3
Measured on a recurring basis | Level 2 | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	254.3	276.1
Measured on a recurring basis | Level 2 | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	167.2	127.5
Measured on a recurring basis | Estimate of Fair Value Measurement
Available-for-sale debt securities:
Total marketable securities	5,609.6	4,528.9
Measured on a recurring basis | Estimate of Fair Value Measurement | Unrestricted
Available-for-sale debt securities:
Equity securities	77.7	48.3
Measured on a recurring basis | Estimate of Fair Value Measurement | Restricted
Available-for-sale debt securities:
Equity securities	924.7	791.5
Measured on a recurring basis | Estimate of Fair Value Measurement | Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,632.2	3,090.3
Measured on a recurring basis | Estimate of Fair Value Measurement | U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	487.8	128.9
Measured on a recurring basis | Estimate of Fair Value Measurement | Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.7	66.3
Measured on a recurring basis | Estimate of Fair Value Measurement | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	254.3	276.1
Measured on a recurring basis | Estimate of Fair Value Measurement | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	$ 167.2	$ 127.5