Share-based Compensation - Activity (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held in employee trust (in shares)	116,368
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (as a percent)	2.85%	2.51%	2.37%
Expected life of option (in years)	3 years 6 months 1 day	4 years 4 months 24 days	3 years 8 months 12 days
Risk-free interest rate (as a percent)	2.48%	1.73%	1.57%
Expected stock price volatility (as a percent)	22.00%	28.10%	26.76%
Options Outstanding
Balance at beginning of year (in shares)	1,548,719	1,737,213	1,595,007
Granted (in shares)	6,000	174,000	445,100
Exercised (in shares)	(316,302)	(292,193)	(192,875)
Lapsed or cancelled (in shares)	(101,400)	(70,301)	(110,019)
Balance at end of period (in shares)	1,137,017	1,548,719	1,737,213
Options exercisable at end of period (in shares)	783,035	800,902	748,617
Weighted average fair value of options granted during the period	$ 84,068	$ 3,816,652	$ 8,363,036
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 101.48	$ 98.44	$ 95.52
Granted (in dollars per share)	97.91	105.66	105.86
Exercised (in dollars per share)	90.22	81.07	76.90
Lapsed or cancelled (in dollars per share)	113.22	121.68	123.86
Balance at end of period (in dollars per share)	135.54	101.48	98.44
Options exercisable at end of period (in dollars per share)	$ 101.28	$ 94.33	$ 87.72
Restricted Stock Awards
Options Outstanding
Weighted average fair value of options granted during the period	$ 13,440,503	$ 9,905,986	$ 10,650,077
Summary of restricted stock
Balance at beginning of year (in shares)	3,298,216	3,202,031	3,137,881
Granted (in shares)	162,900	96,185	98,800
Cancelled (in shares)	(9,100)	0	(34,650)
Balance at end of period (in shares)	3,452,016	3,298,216	3,202,031
Vested during the period (in shares)	92,114	95,736	83,822
Compensation expense recorded	$ 12,757,704	$ 9,809,749	$ 7,153,966

Represents the weighted average fair value of awards granted during the period.

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Exercise Price [Roll Forward]

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Number of common stock held in a trust that has been set up specifically to accumulate stock for the sole purpose of distribution to participating employees but not yet earned.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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- Definition

The 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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- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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