Schedule I - Summary of Investments, Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2025 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 44,882.3
Amount Shown on the Balance Sheet	43,520.8
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	2,125.4	[1]
Amount Shown on the Balance Sheet	2,109.5
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	543.8	[1]
Fair Value of Investments	545.0
Amount Shown on the Balance Sheet	545.0
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	3,450.7	[1]
Fair Value of Investments	3,058.1
Amount Shown on the Balance Sheet	3,058.1
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	1,008.8	[1]
Fair Value of Investments	870.3
Amount Shown on the Balance Sheet	870.3
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	5,048.6	[1]
Fair Value of Investments	4,933.9
Amount Shown on the Balance Sheet	4,933.9
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	1,187.3	[1],[2]
Fair Value of Investments	1,179.3	[2]
Amount Shown on the Balance Sheet	1,179.3	[2]
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	23,500.0	[1]
Fair Value of Investments	22,462.1
Amount Shown on the Balance Sheet	22,462.1
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	8.0	[1]
Fair Value of Investments	7.9
Amount Shown on the Balance Sheet	7.9
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	34,747.2	[1]
Fair Value of Investments	33,056.6
Amount Shown on the Balance Sheet	33,056.6
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	3,668.1
Amount Shown on the Balance Sheet	3,668.1
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	[3]
Amount Shown on the Balance Sheet	69.7	[3]
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	15.2	[4]
Amount Shown on the Balance Sheet	20.0	[4]
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	1,185.8	[4]
Amount Shown on the Balance Sheet	1,456.3	[4]
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	124.4
Amount Shown on the Balance Sheet	124.4
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	3,016.2
Amount Shown on the Balance Sheet	$ 3,016.2

[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] Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types.

[3] Derivatives are carried at fair value.

[4] 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.