Investments (Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ (136.2)	$ 48.6
Less: net gains (losses) recognized on equity securities sold during the period	(0.6)	1.1
Net unrealized gains (losses) recognized during the period on equity securities still held	$ (135.6)	$ 47.5

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=116654751&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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