Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities	$ 4,234.5	$ 3,009.2
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Total marketable securities	618.8	43.6
Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Total marketable securities	4,234.5	3,053.6
Unrestricted | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities		43.6
Equity securities	61.6
Unrestricted | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities		0.0
Equity securities	0.0
Restricted | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Equity securities	557.2	0.0
Restricted | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Equity securities	0.0	44.4
Estimate of Fair Value Measurement | Measured on a recurring basis
Available-for-sale debt securities:
Total marketable securities	4,853.3	3,097.2
Estimate of Fair Value Measurement | Unrestricted | Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities		43.6
Equity securities	61.6
Estimate of Fair Value Measurement | Restricted | Measured on a recurring basis
Available-for-sale debt securities:
Equity securities	557.2	44.4
Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	3,988.1	2,718.4
Corporate bonds | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Corporate bonds | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	3,988.1	2,718.4
Corporate bonds | Estimate of Fair Value Measurement | Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	3,988.1	2,718.4
U.S. government and government agency obligations
Available-for-sale debt securities:
Available-for-sale debt securities	54.4	109.4
U.S. government and government agency obligations | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (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 | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	54.4	109.4
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	54.4	109.4
Sovereign bonds
Available-for-sale debt securities:
Available-for-sale debt securities	27.3	7.6
Sovereign bonds | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Sovereign bonds | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	27.3	7.6
Sovereign bonds | Estimate of Fair Value Measurement | Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	27.3	7.6
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	92.3	113.8
Commercial paper | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Commercial paper | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	92.3	113.8
Commercial paper | Estimate of Fair Value Measurement | Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	92.3	113.8
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	72.4	60.0
Certificates of deposit | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Certificates of deposit | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	72.4	60.0
Certificates of deposit | Estimate of Fair Value Measurement | Measured on a recurring basis
Available-for-sale debt securities:
Available-for-sale debt securities	$ 72.4	$ 60.0