Revenue recognition - Accounts Receivable (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Billed, net	$ 202.2	$ 227.3
Unbilled	56.4	43.4
Total, net	258.6	270.7
Allowance for doubtful accounts receivable	$ 0.5	$ 0.0

Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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Amount of allowance for credit loss on accounts receivable.

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Amounts due for billed services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the Company and, at a minimum, one other party. An example would be amounts billed to customers under contracts or programs but not paid as of the balance sheet date.

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Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

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