Revenue and Contracts with Customers (Contract with Customer Asset and Liability) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Billed accounts receivable	$ 20.1	$ 20.1	$ 50.4
Billed accounts receivable – increase (decrease)		(30.3)
Uranium feed receivable	4.5	4.5	9.8
Uranium feed receivable - increase (decrease)		(5.3)
Unbilled contract revenue	5.6	5.6
Unbilled receivables – increase (decrease)		5.6
Accounts receivable, net	30.2	30.2	60.2
Accounts receivable - increase (decrease)		(30.0)
Deferred costs associated with deferred revenue	138.7	138.7	134.9
Deferred costs – increase (decrease)		3.8
Provision for Loss on Contracts	0.5	0.5
Loss on contracts	0.5	0.5
Deferred revenue	220.5	220.5	204.5
Deferred revenue – increase (decrease)		16.0
Advances from customers	46.7	46.7
Customer advances – increase (decrease)		46.7
Deferred revenue and advances from customers	267.2	267.2	204.5
Advances from customers, noncurrent	$ 29.4	29.4	$ 15.0	$ 14.5
Advances from customers, noncurrent - increase (decrease)		14.4
Deferred costs, additions		3.8
Deferred revenue, additions		$ 16.0

Deferred Cost Additions

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Increase Decrease in Billed Accounts Receivable

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Increase (decrease) in Advances from customers, noncurrent

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Uranium feed receivable

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Uranium feed receivable - increase (decrease)

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Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

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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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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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Amount of deferred revenue recognized for transactions arising during the current reporting period. Deferred revenue is a liability as of the balance sheet date related to a revenue producing activity for which revenue has not yet been recognized. Generally, an Entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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Total carrying amount of consideration received or receivable as of the balance sheet date on potential earnings that were not recognized as revenue or other forms of income in conformity with GAAP, and which are expected to be recognized as such within one year or the normal operating cycle, if longer.

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The noncurrent portion of deferred revenue amount as of balance sheet date. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized, and is not expected to be recognized in the next twelve months. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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The increase (decrease) during the reporting period in the amount due from customers for the credit sale of goods and services; includes accounts receivable and other types of receivables.

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The increase (decrease) during the reporting period in the amount of prepayments by customers for goods or services to be provided at a later date.

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The increase (decrease) during the reporting period in the value of expenditures made during the current reporting period for benefits that will be received over a period of years. Deferred charges differ from prepaid expenses in that they usually extend over a long period of time and may or may not be regularly recurring costs of operation.

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The increase (decrease) during the reporting period, excluding the portion taken into income, in the liability reflecting revenue yet to be earned for which cash or other forms of consideration was received or recorded as a receivable.

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The increase (decrease) during the reporting period of the amount of revenue for work performed for which billing has not occurred, net of uncollectible accounts.

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Losses recognized in the current period on contracts which are expected to generate losses, which are probable and can be reasonably estimated, in a future period.

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Net amount of other deferred costs capitalized at the end of the reporting period. Does not include deferred finance costs or deferred acquisition costs of insurance companies.

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Cumulative provision for contract losses not offset against related costs accumulated on the balance sheet.

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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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