Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value Measurements
(Decrease) increase in fair value of marketable securities	$ (5.2)	$ 2.8
Non-financial assets and liabilities measured at fair value on a nonrecurring basis	$ 0.0		$ 0.0

Amount of increase (decrease) of investment in debt and equity securities, measured at fair value with changes in fair value recognized in net income.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3444-108585

Name:	us-gaap_IncreaseDecreaseInTradingSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration