Revenue Recognition - Comparison to Amounts if ASC 605 Had Been in Effect (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	$ 37,239	$ 32,173	$ 101,065	$ 100,512
Operating expenses	25,414	23,766	76,120	74,552
Operating income	11,825	$ 8,407	24,945	$ 25,960
ASU 2014-09 \| ASC 605 Presentation
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	36,246		98,381
Operating expenses	24,421		73,436
Operating income	11,825		24,945
ASU 2014-09 \| Reclassification
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	(993)		(2,684)
Operating expenses	$ (993)		$ (2,684)

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration