Stock-Based Compensation - Stock Award Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-Based Compensation [Abstract]
Requisite service period (in years)			3 years
Stock-based compensation expense	$ 4.4	$ 4.1	$ 8.9	$ 8.0
Stock-based compensation expense for retirement savings plan contributions	0.8	0.7	1.5	1.4
Associated current and future income tax benefits recognized	0.9	$ 0.8	1.8	$ 1.6
Total unrecognized compensation cost related to non-vested share-based awards	$ 21.9		$ 21.9
Unrecognized compensation cost, weighted-average period for recognition			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	2.0		2.0

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

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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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