Income Taxes - Summary of Changes in Reserve for Uncertain Tax Position (Details) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of fiscal year	$ 127.1
Additions related to purchase accounting	1.0	[1]
Additions for tax positions taken in current year	103.8	[2]
Additions for tax positions taken in prior fiscal years	1.0
Additions for currency translation adjustments	1.4
Reductions as a result of a lapse of the applicable statute of limitations	(3.0)
Balance at end of period	$ 231.3

[1] Adjustment in fiscal 2019 relates to the KapStone Acquisition.

[2] Additions for tax positions taken in current fiscal year include primarily positions taken related to foreign subsidiaries.