Stock-based Compensation - RSU Activity (Details) - 2015 Omnibus Incentive Plan - Restricted Stock Units (RSUs) - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015
Number of Units
Beginning period, outstanding (in shares)	264,000	362,000	307,000	307,000
Units granted (in shares)			70,000	70,000	111,281	390,352
Units vested (in shares)	(8,000)	(95,000)
Units forfeited (in shares)	(6,000)	(3,000)	(15,000)
Ending period, outstanding (in shares)	250,000	264,000	362,000	264,000
Weighted Average Grant-Date Fair Value
Beginning period, outstanding (in dollars per share)	$ 8.91	$ 9.09	$ 9.84	$ 9.84
Units granted (in dollars per share)			5.76
Units vested (in dollars per share)	5.65	9.84
Units forfeited (in dollars per share)	6.13	9.96	8.76
Ending period, outstanding (in dollars per share)	$ 9.08	$ 8.91	$ 9.09	$ 8.91

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
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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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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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

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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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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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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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-Publisher FASB
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- Definition

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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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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