Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 11, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Subsidiary, Sale of Stock [Line Items]
Trading investment, unrealized gain (loss), common stock price variation				$ (7.3)	$ 0.0	$ (7.3)	$ 0.0
Trading investment, unrealized loss, valuation adjustment				69.5	$ 0.0	69.5	$ 0.0
Forward Contracts [Member]
Subsidiary, Sale of Stock [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ 76.8	$ 96.1	$ 76.8	$ 76.8		$ 76.8
Trading investment, unrealized gain (loss)	76.8	96.1	(19.3)
Trading investment, unrealized gain (loss), common stock price variation	(7.3)	(28.1)	20.8
Trading investment, unrealized loss, valuation adjustment	$ 69.5	$ 68.0	$ 1.5
Private Placement [Member] | ROCKWELL AUTOMATION INC. [Member] | PTC [Member]
Subsidiary, Sale of Stock [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		$ 1,000.0
Purchase of stock purchase period		3 years
Sale of stock, subsequent trading period		90 days
Private Placement [Member] | ROCKWELL AUTOMATION INC. [Member] | PTC [Member] | Minimum [Member]
Subsidiary, Sale of Stock [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods		1.00%
Private Placement [Member] | ROCKWELL AUTOMATION INC. [Member] | PTC [Member] | Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods		2.00%