This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Equity Method Investment - Additional Information (Details) - USD ($) $ in Thousands			9 Months Ended
	Mar. 31, 2017	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 05, 2013
Schedule of Equity Method Investments [Line Items]
Equity method investment			$ 124		$ 345
Equity method investment gain on recapitalization of FATHOM™			0	$ 243
FATHOM
Schedule of Equity Method Investments [Line Items]
Equity method investment			124		$ 345	$ 1,600
Ownership percentage of common and preferred units	7.10%	8.00%
Equity method investment gain on recapitalization of FATHOM™			$ 243