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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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The value of the award the counter party seeks in the legal matter.

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Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The value (monetary amount) of the award the plaintiff seeks in the legal matter.

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Amount of cost incurred on construction contract for unapproved change order, claim, or similar item subject to uncertainty.

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