Investments - Allowance for Credit Losses, Unrealized Losses on Securities (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Equity Securities
Investment Holdings [Line Items]
Net unrealized gains (losses) on equity securities still held	$ 39.7	$ 72.4	$ 81.0	$ (42.9)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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