Other Long-Term Assets - Narrative (Details) - Convertible Subordinated Debt - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Convertible Senior Notes at 4.50% February 2022 [Domain]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 120,000,000
Convertible Senior Notes at 5.00% February 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 150,000,000	$ 150,000,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant