Investments - Available-for-sale Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis	$ 547.6		$ 547.6		$ 119.4
Available-for-sale credit losses	(0.1)		0.7	$ 4.3
Credit losses adjustment to amortized cost	$ 0.0	$ 0.0	$ 0.0	$ 3.8
Maximum
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale credit losses		$ 0.1