Deferred Acquisition Costs and Other Actuarial Balances - Retirement and Income Solutions - Balances and changes in DAC (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	$ 3,950.5
Ending balance, Deferred acquisition costs	3,965.5	$ 3,950.5
Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	918.9
Ending balance, Deferred acquisition costs	922.1	918.9
Workplace savings and retirement solutions | Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	506.4	498.0
Costs deferred	12.3	48.1
Amortized to expense	(10.0)	(39.7)
Ending balance, Deferred acquisition costs	508.7	506.4
Individual variable annuities | Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	279.5	278.0
Costs deferred	9.9	27.4
Amortized to expense	(6.7)	(25.9)
Ending balance, Deferred acquisition costs	282.7	279.5
Pension risk transfer | Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	15.4	8.1
Costs deferred	1.9	7.9
Amortized to expense	(0.2)	(0.6)
Ending balance, Deferred acquisition costs	17.1	15.4
Individual fixed deferred annuities | Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	106.1	131.0
Amortized to expense	(5.4)	(24.9)
Ending balance, Deferred acquisition costs	100.7	106.1
Investment only | Retirement and Income Solutions
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	11.5	14.9
Costs deferred	2.7	1.3
Amortized to expense	(1.3)	(4.7)
Ending balance, Deferred acquisition costs	$ 12.9	$ 11.5