Investments (Schedule of Pre-tax Realized and Unrealized Gains and Losses on Equity Securities From Continuing Operations) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 48.6	$ (22.9)
Less: net gains recognized on equity securities sold during the period	1.1	0.5
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 47.5	$ (23.4)

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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