Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Trading securities	$ 0.0	$ 0.2
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Trading securities	0.0	0.2
Significant Other Observable Inputs (Level 2)
Assets
Trading securities	0.0
Unobservable Inputs (Level 3)
Assets
Trading securities	$ 0.0	$ 0.0

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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