Fair Value Measurements - Debt (Details) - Significant Other Observable Inputs (Level 2) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Senior Notes \| 2023 Senior Notes
Debt
Fair value of debt		$ 463.7
Convertible Debt \| 2022 Convertible Notes
Debt
Fair value of debt	$ 294.8	282.9
Convertible Debt \| 2019 Convertible Notes
Debt
Fair value of debt	$ 20.9	$ 21.6

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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