Investments - Derivative - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Embedded Derivative, Fair Value of Embedded Derivative, Net [Abstract]
Aggregate fair value of embedded conversion options	$ 0.9	$ 4.3

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as an asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

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