Common Stock and Stock-Based Awards Plans - Stock-Based Compensation (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares available for issuance	2.2		2.2
Compensation cost recognized for stock-based awards	$ 4.0	$ 3.3	$ 6.9	$ 6.0
Income tax benefit recognized in the income statement for stock-based awards	$ 0.9	$ 0.8	1.6	1.4
Total cash received as a result of stock option exercises			0.3	0.4
Tax benefit realized as a result of stock option exercises and the vesting of other share-based awards			2.5	1.7
Total intrinsic value of stock options exercised			$ 0.5	$ 0.1
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of options			10 years
Performance-Based Restricted Stock Units [Member] \| Employee Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Amount of tax benefit from exercise of option under share-based payment arrangement.

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Amount of cash inflow from issuance of shares under share-based payment arrangement. Includes, but is not limited to, option exercised.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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