Long-Duration Insurance Contracts - Account Balances by Guaranteed Minimum Interest Rates (Details) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,187.0	$ 5,260.6	[1]
Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 886.1	$ 943.6
Less than 2% | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	2.00%	2.00%
Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 382.7	$ 280.6
Equal to 2% but less than 3% | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	2.00%	2.00%
Equal to 2% but less than 3% | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	3.00%	3.00%
Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 609.0	$ 668.2
Equal to 3% but less than 4% | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	3.00%	3.00%
Equal to 3% but less than 4% | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	4.00%	4.00%
Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 2,670.5	$ 2,706.1
Equal to 4% but less than 5% | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	4.00%	4.00%
Equal to 4% but less than 5% | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	5.00%	5.00%
5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 86.9	$ 91.7
5% or higher | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	5.00%	5.00%
Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 4,635.2	$ 4,690.2
At Guaranteed Minimum | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	36.7	262.5
At Guaranteed Minimum | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	162.9	256.1
At Guaranteed Minimum | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	571.3	667.4
At Guaranteed Minimum | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	2,670.5	2,706.1
At Guaranteed Minimum | 5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	86.9	91.7
At Guaranteed Minimum | Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 3,528.3	$ 3,983.8
1-50 Basis Points Above | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0001	0.0001
1-50 Basis Points Above | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0050	0.0050
1-50 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 159.8	$ 370.6
1-50 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	77.9	19.8
1-50 Basis Points Above | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	36.9	0.4
1-50 Basis Points Above | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
1-50 Basis Points Above | 5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
1-50 Basis Points Above | Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 274.6	$ 390.8
51-150 Basis Points Above | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0051	0.0051
51-150 Basis Points Above | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0150	0.0150
51-150 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 489.4	$ 214.4
51-150 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	65.8	4.7
51-150 Basis Points Above | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.7	0.4
51-150 Basis Points Above | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
51-150 Basis Points Above | 5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
51-150 Basis Points Above | Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 555.9	$ 219.5
Greater Than 150 Basis Points Above | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0150	0.0150
Greater Than 150 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 200.2	$ 96.1
Greater Than 150 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	76.1	0.0
Greater Than 150 Basis Points Above | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
Greater Than 150 Basis Points Above | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
Greater Than 150 Basis Points Above | 5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.0	0.0
Greater Than 150 Basis Points Above | Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 276.3	$ 96.1

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements