Incentive Stock Plans - Summary of Performance Shares (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for performance shares granted during year	213,154	226,448	250,584
Weighted average fair value of performance share units granted (in dollars per share)	$ 40.27	$ 32.17	$ 28.79
Intrinsic value of outstanding performance share units	$ 9,229	$ 10,414	$ 7,482
Fair value of performance shares vested	5,670	0	0
Cash used to purchase common shares from current and former employees to pay minimum withholding tax requirements on restricted shares vested	$ 2,651	$ 0	$ 0

Aggregate number of common shares reserved for future issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
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Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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

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