Investments - Schedule of Loans by Ratio (Details) $ in Millions	Mar. 31, 2024 USD ($) security loan	Dec. 31, 2023 USD ($)	Dec. 31, 2023 loan	Dec. 31, 2023 security
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 171.6	$ 144.8
Loan Count	30		28	28
Cost	$ 169.9
Loan Count | loan	29
Equal to or less than 50.0%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 12.3	12.3
Loan Count | security	2			2
Greater than 50.0% to 55.0%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 9.0	9.1
Loan Count | security	2			2
Greater than 55.0% to 60.0%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 20.7	18.9
Loan Count | security	5			4
Greater than 60.0% to 70.0%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 63.2	37.3
Loan Count | security	7			6
Greater than 70.0%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 66.4	67.2
Loan Count | security	14			14
Less than 1.00
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans		36.2
Loan Count | security				8
Cost	$ 36.2
Loan Count | security	8
1.00 to 1.50
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans		29.4
Loan Count | security				6
Cost	$ 54.5
Loan Count | security	7
Greater than 1.5 to 2.0
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans		30.9
Loan Count | security				6
Cost	$ 30.9
Loan Count | security	6
Greater than 2.0 to 3.0
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans		36.0
Loan Count | security				6
Cost	$ 36.0
Loan Count | security	6
Greater than 3.0 to 4.0
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans		$ 12.3
Loan Count | security				2
Cost	$ 12.3
Loan Count | security	2