CONSOLIDATED STATEMENTS OF EQUITY (Parenthetical) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022	Sep. 30, 2021
Shares Issued During Period, New Issues						84,547
Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation		$ (0.7)	$ (0.1)	$ (0.1)	$ (0.4)	$ (0.1)
Issuance of common stock (84,547 shares)						1.7
Net income	$ 46.8	26.9	20.8	39.7	47.8	40.5	$ 94.5	$ 128.0
Issuances of common stock, Value	0.0	(0.1)	0.0	(0.1)	0.0	0.0
Stock-based compensation expense	0.9	1.9	0.6	0.8	1.6	0.4
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,296.5	$ 1,248.8	$ 1,220.6	$ 1,147.9	$ 1,107.4	$ 1,058.4	$ 1,296.5	$ 1,147.9	$ 1,199.3	$ 1,015.9
Common Stock
Common Stock, Shares, Outstanding	49,903,713	49,897,709	49,772,555	49,759,480	49,738,344	49,675,768	49,903,713	49,759,480	49,761,480	49,580,389
Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation		$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Issuance of common stock (84,547 shares)						0.1
Net income	$ 0.0	0.0	0.0	0.0	0.0	0.0
Issuances of common stock, Value	0.0	(0.1)	0.0	(0.1)	0.0	0.0
Stock-based compensation expense	0.0	0.0	0.0	0.0	0.0	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 49.9	$ 49.9	$ 49.8	$ 49.8	$ 49.7	$ 49.7	$ 49.9	$ 49.8	$ 49.8	$ 49.6
Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture	6,004	125,154	11,075	21,136	62,576	10,832
Additional Paid-in Capital
Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation		$ (0.7)	$ (0.1)	$ (0.1)	$ (0.4)	$ (0.1)
Issuance of common stock (84,547 shares)						1.6
Net income	$ 0.0	0.0	0.0	0.0	0.0	0.0
Issuances of common stock, Value	0.0	0.0	0.0	0.0	0.0	0.0
Stock-based compensation expense	0.9	1.9	0.6	0.8	1.6	0.4
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	643.2	642.3	641.1	640.0	639.3	638.1	643.2	640.0	640.6	636.2
Retained Earnings
Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation		0.0	0.0	0.0	0.0	0.0
Issuance of common stock (84,547 shares)						0.0
Net income	46.8	26.9	20.8	39.7	47.8	40.5
Issuances of common stock, Value	0.0	0.0	0.0	0.0	0.0	0.0
Stock-based compensation expense	0.0	0.0	0.0	0.0	0.0	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	602.4	555.6	528.7	457.1	417.4	369.6	602.4	457.1	507.9	329.1
Noncontrolling Interest
Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation		0.0	0.0	0.0	0.0	0.0
Issuance of common stock (84,547 shares)						0.0
Net income	0.0	0.0	0.0	0.0	0.0	0.0
Issuances of common stock, Value	0.0	0.0	0.0	0.0	0.0	0.0
Stock-based compensation expense	0.0	0.0	0.0	0.0	0.0	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0	$ 1.0