Fair Value Measurements and Disclosures - Assets and Liabilities Reported at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair value adjustment of marketable securities and deferred compensation plan liability
Net gain (loss) on marketable securities	$ 1,681	$ (404)
Fair value, measurements, recurring [Member] \| Fair value (Level 1) [Member]
Assets and Liabilities Reported at Fair Value
Marketable securities	$ 24,098		$ 21,779

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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