Stock-Based Compensation - Restricted and Performance Stock Units Rollforward (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock Units
Shares
Nonvested at the beginning of the period (in shares)	334,197	401,716	555,700
Issued (in shares)	22,000	4,020	12,589
Vested (in shares)	(26,399)	(54,215)	(150,073)
Forfeited (in shares)	(4,537)	(17,324)	(16,500)
Nonvested at the end of the period (in shares)	325,261	334,197	401,716	555,700
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 19.45	$ 19.25	$ 19.57
Issued (in dollars per share)	18.25	24.87	23.65
Vested (in dollars per share)	18.98	18.33	20.77
Forfeited (in dollars per share)	21.92	19.68	19.45
Nonvested at the end of the period (in dollars per share)	$ 19.37	$ 19.45	$ 19.25	$ 19.57
Performance Shares
Shares
Nonvested at the beginning of the period (in shares)	159,477	173,851	79,430
Issued (in shares)	29,494	99,466		125,392
Vested (in shares)	(61,919)	(84,417)		(6,915)
Forfeited (in shares)	(11,770)	(29,423)	(24,056)	(24,056)
Nonvested at the end of the period (in shares)	115,282	159,477	173,851	79,430
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 22.99	$ 20.63	$ 18.96
Issued (in dollars per share)	29.51	23.72		$ 21.64
Vested (in dollars per share)	22.63	19.15		19.91
Forfeited (in dollars per share)	25.86	22.52		20.30
Nonvested at the end of the period (in dollars per share)	$ 28.01	$ 22.99	$ 20.63	$ 18.96
Performance Shares Adjustments
Shares
Issued (in shares)	59,091		19,855
Weighted Average Grant-Date Fair Value
Issued (in dollars per share)	$ 26.07		$ 19.02

No definition available.

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- Definition

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

+ 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 (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ 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 (c)(2)(iii)(3)
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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).

+ 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 (c)(2)(iii)(1)
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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).

+ References

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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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.

+ References

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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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-Publisher FASB
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