Stock-Based Compensation - Stock Award Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-Based Compensation [Abstract]
Requisite service period (in years)			3 years
Stock-based compensation expense	$ 4.7	$ 4.3	$ 14.4	$ 13.2
Stock-based compensation expense for retirement savings plan contributions	0.8	0.7	2.5	2.2
Associated current and future income tax benefits recognized	1.5	$ 0.8	4.4	$ 2.6
Total unrecognized compensation cost related to non-vested share-based awards	$ 19.7		$ 19.7
Unrecognized compensation cost, weighted-average period for recognition (in years)			2 years
2012 Long term incentive plan
Stock-Based Compensation [Abstract]
Maximum number of shares authorized under plan (in shares)	7.0		7.0
Number of shares available for issuance under the incentive plan (in shares)	1.7		1.7

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

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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 cost (reversal of cost) for pension and other postretirement benefits.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares authorized for issuance under share-based payment arrangement.

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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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