Investments (Schedule of Pre-tax Realized and Unrealized Gains (Losses) on Equity Securities Recognized in Net Income) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Net gains recognized during the period	$ 23.6	$ 6.7
Less: net losses recognized on equity securities sold during the period	(0.2)	(1.0)
Net unrealized gains recognized during the period on equity securities still held	$ 23.8	$ 7.7

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

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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).

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