Description of Business and Significant Accounting Policies - Investments in Marketable Securities and Inventories and Supplies (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Marketable securities, at fair value	$ 76,362	$ 73,221
Unrealized gains (losses) from marketable securities	$ (800)	$ 1,100
Amortization period of inventories and supplies	24 months

Amount of investment in marketable security, classified as current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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