Revenue - Contract Costs (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenue recognition
Customer relationship, commission costs amortization period	5 years
Practical expedient
Revenue, Practical Expedient, Incremental Cost of Obtaining Contract	true
Deferred commissions
Deferred commissions	$ 31,918	$ 23,802	$ 29,955	$ 21,693
Commission costs capitalized	8,086	6,496
Amortization of deferred commissions	$ 6,124	$ 4,387

The amount of contract cost capitalized during the period.

No definition available.

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- Definition

Customer Relationship, Capitalized Contract Cost, Amortization Period

+ References

No definition available.

+ Details

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- Definition

Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

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- Definition

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

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- References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- Definition

Indicates (true false) whether practical expedient was elected to recognize incremental cost of obtaining contract as expense when incurred if amortization period would have been one year or less.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130569-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL51790836-203054

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- References

No definition available.

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