Share-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement By Share-based Payment Award, Fair Value Assumptions, Percent Expected To Vest	75.60%	75.10%
Stock Issued During Period, Value, Stock Options Exercised		$ 0.1
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	144,231
Executive Officer [Member] \| Restricted Stock [Member] \| 2015 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	100.00%	100.00%
Director [Member] \| Restricted Stock [Member] \| 2004 Non-employee Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage		100.00%
Time Based [Member] \| Executive Officer [Member] \| Restricted Stock [Member] \| 2015 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares (in shares)		44,000
Weighted average grant date fair value		$ 115.39
Time Based [Member] \| Director [Member] \| Restricted Stock [Member] \| 2004 Non-employee Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares (in shares)	10,200	18,000
Weighted average grant date fair value	$ 147.97	$ 113.68
Market Condition [Member] \| Executive Officer [Member] \| Restricted Stock [Member] \| 2015 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares (in shares)	69,000	66,000
Weighted average grant date fair value	$ 125.47	$ 115.39
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Price	$ 165.97	$ 115.39
Exercises in period	0	1,500
Award Date One [Member] \| Time Based [Member] \| Key Employees [Member] \| Restricted Stock [Member] \| 2015 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares (in shares)	51,790
Weighted average grant date fair value	$ 162.42
Award Date Two [Member] \| Time Based [Member] \| Executive Officer [Member] \| Restricted Stock [Member] \| 2015 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares (in shares)	46,000
Weighted average grant date fair value	$ 165.97

Share-based Compensation Arrangement By Share-based Payment Award, Fair Value Assumptions, Percent Expected To Vest

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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 weighted average fair value at grant date for nonvested equity-based awards issued 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 equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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Price of a single share of a number of saleable stocks of a company.

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

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

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Value of stock issued as a result of the exercise of stock options.

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