Investments - Schedule of Fair Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jul. 19, 2018	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	$ 419.0		$ 1,387.6
Equity securities	1,090.0	$ 954.9	0.0
Total investments	1,578.9		1,450.3
Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	169.6		1,005.3
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		20.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	158.4		199.4
Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	65.8		116.8
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	25.2		45.8
Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	1,090.0
Total investments	1,509.0		1,387.6
Fair Value, Measurements, Recurring | Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	169.6		1,005.3
Fair Value, Measurements, Recurring | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fixed income securities			20.3
Fair Value, Measurements, Recurring | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	158.4		199.4
Fair Value, Measurements, Recurring | Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	65.8		116.8
Fair Value, Measurements, Recurring | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	25.2		45.8
Fair Value, Measurements, Recurring | Level 1
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0
Total investments	55.7		98.9
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fixed income securities			0.0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 1 | Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	55.7		98.9
Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 2
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0
Total investments	363.3		1,288.7
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	169.6		1,005.3
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fixed income securities			20.3
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	158.4		199.4
Fair Value, Measurements, Recurring | Level 2 | Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	10.1		17.9
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	25.2		45.8
Fair Value, Measurements, Recurring | Level 3
Debt Securities, Available-for-sale [Line Items]
Equity securities	1,090.0
Total investments	1,090.0		0.0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Fixed income securities			0.0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 3 | Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0		0.0
Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	$ 0.0		$ 0.0