Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Oct. 01, 2019	Sep. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity	$ 1,252.4	$ 1,235.7	$ 1,079.4	$ 1,055.1	$ 1,027.0		$ 769.8
Cumulative Effect, Period of Adoption, Adjustment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity							0.0
Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity	627.1	603.1	481.4	504.4	496.4		599.5
Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity							6.0
Retained Earnings [Member] | Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity						$ 6.0
AOCI Attributable to Parent [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity	$ (55.2)	$ (85.8)	$ (102.8)	$ (142.6)	$ (158.3)		(140.6)
AOCI Attributable to Parent [Member] | Cumulative Effect, Period of Adoption, Adjustment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity							$ (6.0)
AOCI Attributable to Parent [Member] | Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Stockholders' equity						$ 6.0