Accounting Policies - Derivative Instruments (Details) - Convertible Subordinated Debt - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Convertible Senior Notes at 5.00% February 2021
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 61,091,000	$ 150,000,000	$ 150,000,000
Interest rate (as a percent)	5.00%		5.00%
Convertible Senior Notes at 4.50% February 2022
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 120,000,000	$ 120,000,000
Interest rate (as a percent)	4.50%

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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