Stock-Based Compensation Plans - Nonqualified Stock Options, Assumptions and Other Disclosures (Details) - Nonqualified Stock Options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assumptions used to estimate fair value of stock options granted during period
Weighted-average expected volatility (as a percent)	25.70%	23.30%	26.00%
Weighted-average expected term	7 years	7 years	7 years
Weighted-average risk-free interest rate (as a percent)	1.30%	2.60%	2.80%
Weighted-average expected dividend yield (as a percent)	4.33%	4.07%	3.19%
Weighted-average estimated fair value of stock options granted (in dollars per share)	$ 9.64	$ 10.00	$ 14.85
Other nonqualified stock option disclosures
Unrecognized compensation costs	$ 2.1
Weighted-average service period over which unrecognized compensation costs will be recognized	1 year 10 months 24 days
Cash received from stock options exercised	$ 4.4	$ 3.4	$ 29.0
Tax benefits realized for the tax deductions from options exercised	$ 1.4	$ 0.5	$ 3.9

No definition available.

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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 from exercise of option under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of 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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