Investments - Available-for-sale Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis	$ 542.2		$ 119.4
Available-for-sale credit losses	0.2	$ 3.7
Credit losses adjustment to amortized cost	$ 0.0	$ 3.7