Stock-Based Compensation Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Additional Disclosures [Abstract]
Compensation expense, gross	$ 1,917	$ 1,862
Performance-based Shares [Member]
Restricted and Performance Stock Grants [Abstract]
Measuring period for performance-based shares	3 years
Restricted Shares [Member] \| Employees [Member]
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures	5.00%
Restricted Shares [Member] \| Executives [Member]
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures	0.00%
Restricted Shares [Member] \| Directors [Member]
Restricted and Performance Stock Grants [Abstract]
Estimated forfeitures	0.00%
Restricted Shares [Member] \| Age 60 [Member]
Restricted and Performance Stock Grants [Abstract]
Vesting percentage	25.00%
Restricted Shares [Member] \| Age 63 [Member]
Restricted and Performance Stock Grants [Abstract]
Vesting percentage	25.00%
Restricted Shares [Member] \| Age 65 [Member]
Restricted and Performance Stock Grants [Abstract]
Vesting percentage	100.00%
Restricted and Performance-Based Shares [Member]
Restricted and performance-based stock, shares [Roll Forward]
Beginning of period (in shares)	853,958
Granted (in shares)	129
Vested (in shares)	(2,200)
Forfeited (in shares)	(1,575)
End of period (in shares)	850,312
Restricted and performance-based stock, weighted average grant date fair value per share [Roll Forward]
Beginning of period (in dollars per share)	$ 33.25
Granted (in dollars per share)	41.18
Vested (in dollars per share)	41.71
Forfeited (in dollars per share)	41.44
End of period (in dollars per share)	$ 33.21
Additional Disclosures [Abstract]
Compensation expense, gross	$ 1,900	1,800
Compensation expense, net of tax	1,400	$ 1,100
Unamortized compensation expense	$ 14,700
Restricted and Performance-Based Shares [Member] \| Employees [Member]
Additional Disclosures [Abstract]
Weighted average period of recognition for unrecognized compensation expense	4 years 6 months
Restricted and Performance-Based Shares [Member] \| Directors [Member]
Additional Disclosures [Abstract]
Weighted average period of recognition for unrecognized compensation expense	1 month 6 days
Restricted and Performance-Based Shares [Member] \| Executives and Directors [Member]
Restricted and Performance Stock Grants [Abstract]
Expiration of vesting period	3 years
Restricted and Performance-Based Shares [Member] \| Executives and Directors [Member] \| Minimum [Member]
Restricted and Performance Stock Grants [Abstract]
Post vesting holding period for restricted and performance shares issued	1 year
Restricted and Performance-Based Shares [Member] \| Executives and Directors [Member] \| Maximum [Member]
Restricted and Performance Stock Grants [Abstract]
Post vesting holding period for restricted and performance shares issued	2 years

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Represents the percentage of the forfeitures on share-based payment award during the period.

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Post vesting holding period that once the award vests the participant needs to hold the award for a given time period until they can exercise the award.

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The amount of expense, net of income tax, 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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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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-Publisher FASB
-Name Accounting Standards Codification
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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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-Publisher FASB
-Name Accounting Standards Codification
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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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-Name Accounting Standards Codification
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
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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 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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-Name Accounting Standards Codification
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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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Percentage of vesting of share-based compensation awards.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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