Investments - Narrative (Details) - USD ($)		2 Months Ended	12 Months Ended
	Jul. 19, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Debt and Equity Securities, FV-NI [Line Items]
Pre-tax gross unrealized losses on available-for-sale investments			$ 2,700,000
Purchases of investments, amount outstanding		$ 0	0
Trading investment, fair value	$ 954,900,000	1,090,000,000	1,090,000,000	$ 0
Change in fair value of investments			90,000,000	0	$ 0
Gain on investments			123,700,000	0	0
Trading investment, unrealized gain (loss), valuation adjustment			33,700,000	$ 0	$ 0
PTC
Debt and Equity Securities, FV-NI [Line Items]
Number of shares purchased (in shares)	10,582,010
Purchase of stock price (usd per share)	$ 94.50
Consideration paid to purchase stock	$ 1,000,000,000
Entity specific transfer restrictions, period	3 years
Sale of stock, subsequent trading period	90 days
Trading investment, fair value		1,090,000,000	1,090,000,000
Change in fair value of investments	$ (45,100,000)	135,100,000	90,000,000
Gain on investments	26,800,000	96,900,000	123,700,000
Trading investment, unrealized gain (loss), valuation adjustment	$ 71,900,000	$ 38,200,000	$ 33,700,000
PTC | Minimum
Debt and Equity Securities, FV-NI [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	1.00%
PTC | Maximum
Debt and Equity Securities, FV-NI [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	2.00%