Schedule I Summary of Investments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	$ 45,491.4	[1]
Amount Shown on the Balance Sheet	51,794.3
United States Government and Government Agencies and Authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	983.5	[1]
Fair Value of Investments	1,238.5
Amount Shown on the Balance Sheet	1,238.5
States, Municipalities, and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	1,745.0	[1]
Fair Value of Investments	2,121.5
Amount Shown on the Balance Sheet	2,121.5
Foreign Government Debt
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	1,101.1	[1]
Fair Value of Investments	1,307.4
Amount Shown on the Balance Sheet	1,307.4
Public Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	7,046.1	[1]
Fair Value of Investments	8,550.6
Amount Shown on the Balance Sheet	8,550.6
Mortgage/Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	2,224.9	[1]
Fair Value of Investments	2,431.8
Amount Shown on the Balance Sheet	2,431.8
All Other Corporate Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	25,658.8	[1]
Fair Value of Investments	29,365.2
Amount Shown on the Balance Sheet	29,365.2
Redeemable Preferred Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	44.0	[1]
Fair Value of Investments	49.9
Amount Shown on the Balance Sheet	49.9
Fixed Maturity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	38,803.4	[1]
Fair Value of Investments	45,064.9
Amount Shown on the Balance Sheet	45,064.9
Mortgage Loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	1,856.6	[1]
Amount Shown on the Balance Sheet	1,856.6
Policy Loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	3,306.6	[1]
Amount Shown on the Balance Sheet	3,306.6
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	0	[1]
Amount Shown on the Balance Sheet	28.0	[2]
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	7.9	[1]
Amount Shown on the Balance Sheet	12.5
Miscellaneous Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	542.6	[1]
Amount Shown on the Balance Sheet	551.4	[3]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost or Amortized Cost of Investments	974.3	[1]
Amount Shown on the Balance Sheet	$ 974.3

[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.