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