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, 2020 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 45,064.3	[1]
Amount Shown on the Balance Sheet	52,683.5
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	559.0	[1]
Fair Value of Investments	709.8
Amount Shown on the Balance Sheet	709.8
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	3,609.9	[1]
Fair Value of Investments	4,261.2
Amount Shown on the Balance Sheet	4,261.2
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	902.9	[1]
Fair Value of Investments	1,168.2
Amount Shown on the Balance Sheet	1,168.2
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	5,486.4	[1]
Fair Value of Investments	6,962.3
Amount Shown on the Balance Sheet	6,962.3
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	1,019.9	[1]
Fair Value of Investments	1,107.7
Amount Shown on the Balance Sheet	1,107.7
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	24,958.8	[1]
Fair Value of Investments	29,918.6
Amount Shown on the Balance Sheet	29,918.6
Redeemable Preferred Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	9.6	[1]
Fair Value of Investments	9.5
Amount Shown on the Balance Sheet	9.5
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	36,546.5	[1]
Fair Value of Investments	44,137.3
Amount Shown on the Balance Sheet	44,137.3
Mortgages
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,445.2	[1]
Amount Shown on the Balance Sheet	2,432.1
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	3,683.9	[1]
Amount Shown on the Balance Sheet	3,683.9
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	[1]
Amount Shown on the Balance Sheet	19.8	[2]
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	32.6	[1]
Amount Shown on the Balance Sheet	28.3	[3]
Private Equity Partnerships
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	720.9
Amount Shown on the Balance Sheet	747.5	[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	165.2	[1]
Amount Shown on the Balance Sheet	164.6	[3]
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	1,470.0	[1]
Amount Shown on the Balance Sheet	$ 1,470.0

[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 expected 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 equity securities primarily results from changes in the partnership owner's equity and the issuer's equity since acquisition, respectively.