Benefit Plans - Shares Granted to Certain Employees and Trustees (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	5 years
Non-vested Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for services (in shares)	237,560	225,090
Value per share granted (usd per share)	$ 6.83	$ 5.23
Shares issued for services, value	$ 2,551	$ 1,724
Vesting period (in years)	3 years
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Measurement period (in years)	3 years
Share-based Compensation Award, Tranche One [Member] \| Index Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for services (in shares)	106,706	404,466
Value per share granted (usd per share)	$ 6.82	$ 4.53
Share-based Compensation Award, Tranche One [Member] \| Peer Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for services (in shares)	106,705	404,463
Value per share granted (usd per share)	$ 6.34	$ 4.58
Share-based Compensation Award, Tranche Two [Member] \| Index Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for services (in shares)	163,466	0
Value per share granted (usd per share)	$ 4.05	$ 0.00
Share-based Compensation Award, Tranche Two [Member] \| Peer Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for services (in shares)	163,463	0
Value per share granted (usd per share)	$ 4.27	$ 0.00

Share-based Compensation Arrangement by Share-based Payment Award, Measurement Period

No definition available.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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).

+ 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)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Number of shares issued in lieu of cash for services contributed to the entity. Number of shares includes, but is not limited to, shares issued for services contributed by vendors and founders.

+ References

No definition available.

+ Details

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- Definition

Value of stock issued in lieu of cash for services contributed to the entity. Value of the stock issued includes, but is not limited to, services contributed by vendors and founders.

+ References

No definition available.

+ Details

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