Fair Value Measurements - Unrealized Gain (Loss) on Investments (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Fair Value Disclosures [Abstract]
Net unrealized gains included in earnings related to investments	$ 18.8

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration