Deposit Asset on Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2019	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Effects of Reinsurance
Reinsurance block of in-force fixed and variable annuity business			$ 3,200.0
Reinsured percentage of total in force fixed annuity account balances			50.00%
Fixed annuity reserves reinsured on a coinsurance basis	$ 2,400.0			$ 2,400.0
Separate account asset	3,326.8			3,326.8		$ 2,891.4
Separate account liability	3,326.8			3,326.8		$ 2,891.4
Net investment income	103.7	$ 93.7		308.4	$ 256.4
Deposit asset on reinsurance
Effects of Reinsurance
Net investment income	25.6	$ 24.5		75.1	$ 72.1
Reinsurance Contract Modified Coinsurance Basis
Effects of Reinsurance
Separate account asset	800.0			800.0
Separate account liability	$ 800.0			$ 800.0
Minimum
Effects of Reinsurance
Minimum crediting rating			4.50%