Investments - Commercial Mortgage Loans (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 171.6	$ 144.8
Total Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	221.0	144.8
Mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	171.6	144.8
Private loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	49.4	0.0
Fair Value | Total Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	225.3	148.8
Fair Value | Mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	176.0	148.8
Fair Value | Private loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans	$ 49.3	$ 0.0