Share Based Award Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock and Performance Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested and expected to vest	285,503
Shares granted	93,442
Change in units due to performance expectations	41,372
Vested	(71,868)
Forfeited	(7,745)
Nonvested and expected to vest	340,704	285,503
Nonvested, Weighted Average Grant Date Fair Value	$ 155.27	$ 116.28
Grants in period, Grant date fair value per share	251.19
Change in units due to performance expectations	154.75
Vested Intrinsic Value, Amount Per Share	122.00
Forfeited, Weighted Average Grant Date Fair Value	$ 181.07
Remaining Contractual Terms	1 year
Aggregate intrinsic value	$ 78,236
Performance Restricted Stock Units (PRSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	30,348	20,686	36,370
Grants in period, Grant date fair value per share	$ 248.65	$ 154.75	$ 86.47
Grant date fair value performance restricted stock units	$ 7,546	$ 3,201	$ 3,145
Intrinsic value vested	$ 0	$ 0	$ 0
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance Shares Earned	0	0	0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	63,094	107,678	60,377
Grants in period, Grant date fair value per share	$ 252.42	$ 156.49	$ 87.47
Grant date fair value of restricted stock units granted	$ 15,926	$ 16,851	$ 5,281
Intrinsic value vested	$ 17,086	$ 9,813	$ 4,680

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Grant date fair value of restricted stock units granted

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Performance Shares Earned

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ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceExpectationAdditionsIntrinsicValue

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- Definition

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

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Per share weighted average intrinsic value of equity-based compensation awards forfeited. Excludes stock and unit options.

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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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- Definition

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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Per share weighted average intrinsic value of equity-based compensation awards vested. Excludes stock and unit options.

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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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Intrinsic value of equity-based compensation awards not vested. Excludes stock and unit options.

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Intrinsic value of equity-based compensation awards vested. Excludes stock and unit options.

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