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, 2015 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	$ 46,274.1	[1]
Amount Shown on the Balance Sheet	50,023.7
United States Government and Government Agencies and Authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,265.8	[1]
Fair Value of Investments	1,465.2
Amount Shown on the Balance Sheet	1,465.2
States, Municipalities, and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,828.3	[1]
Fair Value of Investments	2,117.1
Amount Shown on the Balance Sheet	2,117.1
Foreign Governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	897.2	[1]
Fair Value of Investments	1,051.6
Amount Shown on the Balance Sheet	1,051.6
Public Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	6,979.3	[1]
Fair Value of Investments	8,020.4
Amount Shown on the Balance Sheet	8,020.4
Mortgage/Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	2,318.6	[1]
Fair Value of Investments	2,481.5
Amount Shown on the Balance Sheet	2,481.5
All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	26,325.5	[1]
Fair Value of Investments	28,171.4
Amount Shown on the Balance Sheet	28,171.4
Redeemable Preferred Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	44.0	[1]
Fair Value of Investments	47.2
Amount Shown on the Balance Sheet	47.2
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	39,658.7	[1]
Fair Value of Investments	43,354.4
Amount Shown on the Balance Sheet	43,354.4
Mortgages
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1,883.6	[1]
Amount Shown on the Balance Sheet	1,883.6
Policy Loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	3,395.4	[1]
Amount Shown on the Balance Sheet	3,395.4
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	0.0	[1]
Amount Shown on the Balance Sheet	49.8	[2]
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	1.2	[1]
Amount Shown on the Balance Sheet	1.4
Miscellaneous Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	527.9	[1]
Amount Shown on the Balance Sheet	531.8	[3]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost of Investments	807.3	[1]
Amount Shown on the Balance Sheet	$ 807.3

[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.