Investments - Equity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total equity securities	$ 72.4	$ 92.5
Common stock
Debt Securities, Available-for-sale [Line Items]
Total equity securities	9.7	10.5
Perpetual preferred stock
Debt Securities, Available-for-sale [Line Items]
Total equity securities	$ 62.7	$ 82.0