Business and Basis of Presentation - Unrestricted and Restricted Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Trading securities, unrestricted	$ 25,081
Trading securities, restricted	2,275
Held-to-maturity securities	$ 16,374	$ 29,137

Amount of investment in debt security measured at amortized cost (held-to-maturity), classified as noncurrent.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26626-111562

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