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Commitments and Contingencies (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jan. 31, 2021
Apr. 30, 2019
Dec. 31, 2019
Apr. 30, 2019
Mar. 31, 2019
Sep. 30, 2022
Sep. 30, 2021
Dec. 06, 2019
Dec. 31, 2014
New York                  
Commitments and Contingencies                  
Percentage of claim recoveries due to seller 90.00%                
Percentage of claim recoveries due to purchaser 10.00%                
New York | Maximum                  
Commitments and Contingencies                  
Contingency liability           $ 146.0      
Montana                  
Commitments and Contingencies                  
Damages sought   $ 132.0   $ 132.0 $ 79.0        
Damages sought, counter party     $ 93.0            
Department of Energy | New York | Minimum                  
Commitments and Contingencies                  
Contingency liability           106.0      
Department of Energy | New York | Maximum                  
Commitments and Contingencies                  
Contingency liability           146.0      
Fund investment capital commitments                  
Commitments and Contingencies                  
Commitment           $ 13.2      
Commitment period           6 years      
Standby letters of credit                  
Commitments and Contingencies                  
Outstanding standby letters of credit           $ 640.3 $ 478.5    
Unsecured credit facilities           427.4      
Contingency liability           644.7      
Surety Bond                  
Commitments and Contingencies                  
Contingency liability           4,400.0      
Collectibility of Receivables                  
Commitments and Contingencies                  
Unapproved change orders                 $ 103.0
Collectibility of Receivables | New York                  
Commitments and Contingencies                  
Unapproved change orders               $ 60.4  
Collectibility of Receivables | New York | Minimum                  
Commitments and Contingencies                  
Damages sought           148.5      
Collectibility of Receivables | New York | Maximum                  
Commitments and Contingencies                  
Damages sought           329.4      
Collectibility of Receivables | Montana                  
Commitments and Contingencies                  
Unapproved change orders           90.0      
Damages sought           $ 144.0