Summary of Significant Accounting Policies Stock-Based Compensation (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance	34,300,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Minimum exercise price of stock option as a percent of common stock fair value on the grant date	100.00%
Period after the grant date during which stock options can't be exercised (in months)	6 months
Maximum term of awards (in years)	10 years
Stock options granted (in shares)	554,594	476,418	710,710
Estimated weighted-average fair value per stock option (in dollars per share)	$ 10.94	$ 8.60	$ 7.71
Risk-free interest rate, minimum (as a percent)	0.20%	2.50%	1.60%
Risk-free interest rate, maximum (as a percent)	1.90%	2.70%	2.80%
Dividend yield (as a percent)	3.00%	3.60%	3.50%
Expected volatility (as a percent)	16.30%	17.00%	18.00%
Expected life (in years)	8 years 7 months 6 days	8 years 6 months	5 years 10 months 24 days
Restricted stock \| Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Percentage to vest each year after grant date	33.00%
Restricted stock \| Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year
Performance units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Performance units \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payout ratio (as a percent)	0.00%
Performance units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payout ratio (as a percent)	175.00%

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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- Definition

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

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

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

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Gross number of share options (or share units) granted during the period.

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- Definition

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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Percentage of vesting of award under share-based payment arrangement.

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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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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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- Definition

Minimum Exercise Price of Stock Option as a Percent of Common Stock Fair Value on the Grant Date

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Period during which stock options can't be exercised except in the event of a change in control.

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- Definition

Payout percentage applied to the number of outstanding performance units to determine the final payout out under an equity-based compensation plan.

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