Leases (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Oct. 01, 2019	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Operating lease payments, due remainder of fiscal year	$ 1.7		$ 1.7
Operating lease payments, due year two	2.8		2.8
Operating lease payments, due year three	2.8		2.8
Operating lease payments, due year four	2.5		2.5
Operating lease payments, due year five	2.0		2.0
Operating lease payments, due after year five	4.8		4.8
Operating lease payments, total due	16.6		16.6
Operating lease liability, undiscounted excess amount	(2.0)		(2.0)
Operating lease liability	14.6		14.6
Operating lease payments, due remainder of fiscal year						$ 2.7
Operating lease payments, due year two						2.7
Operating lease payments, due year three						2.7
Operating lease payments, due year four						2.7
Operating lease payments, due year five						1.9
Operating lease payments, due after year five						4.7
Operating lease payments, total due						$ 17.4
Operating lease expense	$ 0.9		2.0
Operating lease payments			$ 1.8
Operating lease, weighted average remaining lease term	6 years		6 years
Operating lease, weighted average discount rate, percent	4.20%		4.20%
Operating lease, rent expense		$ 0.9		$ 1.6
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	1 year		1 year
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	7 years		7 years
Other assets
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset	$ 13.4		$ 13.4
Other current liabilities
Lessee, Lease, Description [Line Items]
Operating lease liability, current	2.6		2.6
Other noncurrent liabilities
Lessee, Lease, Description [Line Items]
Operating lease liability, noncurrent	12.0		12.0
Other liabilities
Lessee, Lease, Description [Line Items]
Operating lease liability	$ 14.6		$ 14.6
Accounting Standards Update 2016-02 | Other assets
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset					$ 14.8
Accounting Standards Update 2016-02 | Other liabilities
Lessee, Lease, Description [Line Items]
Operating lease liability					$ 16.0