Investments - Equity Securities Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Net gains recognized during the period on equity securities	$ 578.9	$ 1,263.8
Less: net gains recognized during the period on equity securities sold during the period	(53.0)	(180.8)
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date	$ 525.9	$ 1,083.0