Schedule I - Summary of Investments, Other Than Investments in Related Parties Schedule I - Summary of Investments, Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2023 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 47,253.7
Amount Shown on the Balance Sheet	45,962.4
Commercial Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,328.4	[1]
Amount Shown on the Balance Sheet	2,318.2
United States Government and Government Agencies and Authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	618.6	[1]
Fair Value of Investments	624.8
Amount Shown on the Balance Sheet	624.8
States, Municipalities, and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	4,041.3	[1]
Fair Value of Investments	3,678.4
Amount Shown on the Balance Sheet	3,678.4
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	982.1	[1]
Fair Value of Investments	890.7
Amount Shown on the Balance Sheet	890.7
Public Utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	5,398.2	[1]
Fair Value of Investments	5,321.6
Amount Shown on the Balance Sheet	5,321.6
Mortgage/Asset-backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	658.0	[1]
Fair Value of Investments	644.1
Amount Shown on the Balance Sheet	644.1
All Other Corporate Bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	26,708.4	[1]
Fair Value of Investments	25,670.7
Amount Shown on the Balance Sheet	25,670.7
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	4.0	[1]
Fair Value of Investments	3.6
Amount Shown on the Balance Sheet	3.6
Fixed Maturities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	38,410.6	[1]
Fair Value of Investments	36,833.9
Amount Shown on the Balance Sheet	36,833.9
Policy Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	3,620.2
Amount Shown on the Balance Sheet	3,620.2
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	0.0
Amount Shown on the Balance Sheet	99.9	[2]
Perpetual Preferred Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	27.8
Amount Shown on the Balance Sheet	31.9	[3]
Private Equity Funds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,134.6
Amount Shown on the Balance Sheet	1,326.2	[3]
Miscellaneous Long-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	121.4
Amount Shown on the Balance Sheet	121.4
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,610.7
Amount Shown on the Balance Sheet	$ 1,610.7

[1] The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from declines in fair value, amortization of premiums, and/or accretion of discounts. The amortized cost for these investments does not include allowance for credit losses.

[2] Derivatives are carried at fair value.

[3] The difference between amortized cost and carrying value for private equity partnerships and perpetual preferred and equity securities primarily results from changes in the partnership owner's equity and the security's market valuation since acquisition, respectively.