Long-Duration Insurance Contracts - Account Balances by Guaranteed Minimum Interest Rates (Details) $ in Millions	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 5,119.0	$ 5,187.0
Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	836.4	886.1
Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	452.2	382.7
Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	577.4	609.0
Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	2,635.7	2,670.5
5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	84.3	86.9
Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 4,586.0	$ 4,635.2
Minimum | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	2.00%	2.00%
Minimum | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	3.00%	3.00%
Minimum | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	4.00%	4.00%
Minimum | 5% or higher
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	5.00%	5.00%
Maximum | Less than 2%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	2.00%	2.00%
Maximum | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	3.00%	3.00%
Maximum | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	4.00%	4.00%
Maximum | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholder account balance, guaranteed minimum credit rating	5.00%	5.00%
At Guaranteed Minimum | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 19.5	$ 36.7
At Guaranteed Minimum | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	124.1	162.9
At Guaranteed Minimum | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	534.6	571.3
At Guaranteed Minimum | Equal to 4% but less than 5%
Policyholder Account Balance [Line Items]
Policyholders' account balances	2,635.7	2,670.5
At Guaranteed Minimum | 5% or higher
Policyholder Account Balance [Line Items]
Policyholders' account balances	84.3	86.9
At Guaranteed Minimum | Policyholder account balance with a guaranteed minimum crediting rate
Policyholder Account Balance [Line Items]
Policyholders' account balances	3,398.2	3,528.3
1-50 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	102.4	159.8
1-50 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	110.9	77.9
1-50 Basis Points Above | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	42.2	36.9
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	$ 255.5	$ 274.6
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
51-150 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 463.3	$ 489.4
51-150 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	85.1	65.8
51-150 Basis Points Above | Equal to 3% but less than 4%
Policyholder Account Balance [Line Items]
Policyholders' account balances	0.6	0.7
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	$ 549.0	$ 555.9
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
Greater Than 150 Basis Points Above | Less than 2%
Policyholder Account Balance [Line Items]
Policyholders' account balances	$ 251.2	$ 200.2
Greater Than 150 Basis Points Above | Equal to 2% but less than 3%
Policyholder Account Balance [Line Items]
Policyholders' account balances	132.1	76.1
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	$ 383.3	$ 276.3
Greater Than 150 Basis Points Above | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance, above guaranteed minimum crediting rate	0.0150	0.0150