Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale debt securities:
Available-for-sale debt securities	$ 4,337.6	$ 3,689.1
Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,706.7	3,090.3
U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	178.0	128.9
Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.5	66.3
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	215.9	276.1
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	171.5	127.5
Measured on a recurring basis
Available-for-sale debt securities:
Total marketable securities	5,738.9	4,528.9
Measured on a recurring basis | Unrestricted
Available-for-sale debt securities:
Equity securities	30.4	48.3
Measured on a recurring basis | Restricted
Available-for-sale debt securities:
Equity securities	1,370.9	791.5
Measured on a recurring basis | Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,706.7	3,090.3
Measured on a recurring basis | U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	178.0	128.9
Measured on a recurring basis | Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.5	66.3
Measured on a recurring basis | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	215.9	276.1
Measured on a recurring basis | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	171.5	127.5
Measured on a recurring basis | Level 1
Available-for-sale debt securities:
Total marketable securities	1,383.3	839.8
Measured on a recurring basis | Level 1 | Unrestricted
Available-for-sale debt securities:
Equity securities	30.4	48.3
Measured on a recurring basis | Level 1 | Restricted
Available-for-sale debt securities:
Equity securities	1,352.9	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	4,355.6	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	18.0	0.0
Measured on a recurring basis | Level 2 | Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,706.7	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	178.0	128.9
Measured on a recurring basis | Level 2 | Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	65.5	66.3
Measured on a recurring basis | Level 2 | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	215.9	276.1
Measured on a recurring basis | Level 2 | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	$ 171.5	$ 127.5