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Equity Incentive Plan - Share and unit activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding, Unvested restricted stock at beginning of the period	212,000	227,000	228,000
Shares Outstanding, Restricted stock granted	54,488	57,247	88,466
Shares Outstanding, Restricted stock vested	(70,000)	(71,000)	(78,000)
Shares Outstanding, Restricted stock forfeited	(2,000)	(1,000)	(11,000)
Shares Outstanding, Unvested restricted stock at end of the period	194,000	212,000	227,000
Weighted Average Grant Date Fair Value, Restricted stock granted (in dollars per share)	$ 65.85	$ 48.85	$ 48.59
Weighted Average Grant Date Fair Value, Restricted stock vested (in dollars per share)	39.55	36.06	30.95
Weighted Average Grant Date Fair Value, Restricted stock forfeited (in dollars per share)	54.08	48.28	39.68
Weighted Average Grant Date Fair Value, Unvested restricted stock at end of the period (in dollars per share)	$ 50.71	$ 42.74	$ 39.47	$ 33.02
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding, Unvested restricted stock at beginning of the period	31
Shares Outstanding, Restricted stock granted		31
Shares Outstanding, Unvested restricted stock at end of the period		31
Weighted Average Grant Date Fair Value, Restricted stock granted (in dollars per share)		$ 47.73
Weighted Average Grant Date Fair Value, Unvested restricted stock at end of the period (in dollars per share)		$ 47.73
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding, Restricted stock granted	30
Weighted Average Grant Date Fair Value, Restricted stock granted (in dollars per share)	$ 65.66
Performance Units And Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding, Unvested restricted stock at end of the period	61
Weighted Average Grant Date Fair Value, Unvested restricted stock at end of the period (in dollars per share)	$ 56.57

X

- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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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-Publisher FASB
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- Definition

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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- Definition

Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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