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, 2018 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	$ 47,914.2	[1]
Amount Shown on the Balance Sheet	50,707.6
U.S. Government and Agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	1,702.1	[1]
Fair Value of Investments	1,814.4
Amount Shown on the Balance Sheet	1,814.4
State and Municipal Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,121.5	[1]
Fair Value of Investments	2,424.2
Amount Shown on the Balance Sheet	2,424.2
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	825.8	[1]
Fair Value of Investments	983.7
Amount Shown on the Balance Sheet	983.7
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	6,626.2	[1]
Fair Value of Investments	7,412.8
Amount Shown on the Balance Sheet	7,412.8
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	1,523.8	[1]
Fair Value of Investments	1,582.7
Amount Shown on the Balance Sheet	1,582.7
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	27,436.8	[1]
Fair Value of Investments	28,754.0
Amount Shown on the Balance Sheet	28,754.0
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	39.0	[1]
Fair Value of Investments	39.9
Amount Shown on the Balance Sheet	39.9
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	40,275.2	[1]
Fair Value of Investments	43,011.7
Amount Shown on the Balance Sheet	43,011.7
Mortgages
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost of Investments	2,295.0	[1]
Amount Shown on the Balance Sheet	2,295.0
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	3,729.9	[1]
Amount Shown on the Balance Sheet	3,729.9
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	30.9	[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	29.2
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	613.4	[1]
Amount Shown on the Balance Sheet	642.8	[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	968.1	[1]
Amount Shown on the Balance Sheet	$ 968.1

[1] The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and/or accretion of discounts.

[2] Derivatives are carried at fair value.

[3] The difference between amortized cost and carrying value primarily results from changes in the partnership owner's equity since acquisition.