Fair Value of Financial Instruments - Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Liabilities
Contingent obligation due selling unitholders of Cogent	$ 18,293	$ 13,763
Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	72,193	183,369
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	72,193	183,369
Liabilities
Contingent obligation due selling unitholders of Cogent	18,293	13,763
Liabilities	18,293	13,763
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	72,193	183,369
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	72,193	183,369
Liabilities
Contingent obligation due selling unitholders of Cogent	0	0
Liabilities	0	0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	0	0
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	0	0
Liabilities
Contingent obligation due selling unitholders of Cogent	18,293	0
Liabilities	18,293	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	0	0
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	0	0
Liabilities
Contingent obligation due selling unitholders of Cogent	0	13,763
Liabilities	$ 0	$ 13,763

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value of financial and nonfinancial obligations.

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