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Commitments and Contingencies (Details) - USD ($) $ in Millions |  |  |  | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jan. 31, 2020 | Oct. 31, 2019 | Apr. 30, 2019 | Apr. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Dec. 06, 2019 | Dec. 31, 2014
New York |  |  |  |  |  |  |  |
Commitments and Contingencies |  |  |  |  |  |  |  |
Contingency liability |  |  |  |  |  | $ 146.0 |  |
Percentage of claim recoveries due to seller | 10.00% |  |  |  |  |  |  |
Percentage of claim recoveries due to purchaser | 90.00% |  |  |  |  |  |  |
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 |  |
Capital Addition Purchase Commitments |  |  |  |  |  |  |  |
Commitments and Contingencies |  |  |  |  |  |  |  |
Commitment |  |  |  |  |  | $ 22.1 |  |
Commitment period |  |  |  |  |  | 8 years |  |
Standby letters of credit |  |  |  |  |  |  |  |
Commitments and Contingencies |  |  |  |  |  |  |  |
Contingency liability |  |  |  |  |  | $ 529.1 |  |
Surety Bond |  |  |  |  |  |  |  |
Commitments and Contingencies |  |  |  |  |  |  |  |
Contingency liability |  |  |  |  |  | 6,200.0 |  |
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 |  |

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