Summary of Significant Accounting Policies - Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	$ 282.6	$ 204.2	$ 257.5	$ 244.0	$ 214.5	$ 237.6	$ 216.5	$ 185.8	$ 988.3	$ 854.4	$ 827.5
Cost of goods sold									650.3	542.6	549.8
Gross profit	$ 89.8	$ 68.7	$ 88.2	$ 91.3	$ 76.7	$ 90.5	$ 79.0	$ 65.6	338.0	311.8	277.7
Selling, general and administrative expenses									151.8	127.1	135.1
Amortization of intangible assets									22.2	22.2	22.8
Operating profit									$ 164.0	162.5	$ 119.8
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										863.2
Cost of goods sold										542.6
Gross profit										320.6
Selling, general and administrative expenses										135.9
Amortization of intangible assets										22.2
Operating profit										162.5
Difference between revenue guidance in effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										(8.8)
Cost of goods sold										0.0
Gross profit										(8.8)
Selling, general and administrative expenses										(8.8)
Amortization of intangible assets										0.0
Operating profit										$ 0.0