Schedule I. Summary of Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 23,397.8
Value	26,157.9	[1]
Amount Shown on Consolidated Balance Sheets	26,083.7
U.S. Treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,011.5
Value	1,146.6	[1]
Amount Shown on Consolidated Balance Sheets	1,146.6
U.S. government agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	379.4
Value	397.0	[1]
Amount Shown on Consolidated Balance Sheets	397.0
State, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	77.2
Value	93.1	[1]
Amount Shown on Consolidated Balance Sheets	93.1
U.S. corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,438.0
Value	10,574.3	[1]
Amount Shown on Consolidated Balance Sheets	10,574.3
Foreign securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,009.7	[2]
Value	5,552.0	[1],[2]
Amount Shown on Consolidated Balance Sheets	5,552.0	[2]
Residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,070.4
Value	2,357.5	[1]
Amount Shown on Consolidated Balance Sheets	2,357.5
Commercial mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	748.7
Value	839.1	[1]
Amount Shown on Consolidated Balance Sheets	839.1
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	475.7
Value	495.6	[1]
Amount Shown on Consolidated Balance Sheets	495.6
Fixed maturities pledged, included in securities pledged
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	19,210.6
Value	21,455.2	[1]
Amount Shown on Consolidated Balance Sheets	21,455.2
Equity securities, available-for-sale
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	129.3
Value	142.8	[1]
Amount Shown on Consolidated Balance Sheets	142.8
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,872.7
Value	2,946.9	[1]
Amount Shown on Consolidated Balance Sheets	2,872.7
Policy loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	240.9
Value	240.9	[1]
Amount Shown on Consolidated Balance Sheets	240.9
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	679.8
Value	679.8	[1]
Amount Shown on Consolidated Balance Sheets	679.8
Limited Partnerships/corporations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	179.6
Value	179.6	[1]
Amount Shown on Consolidated Balance Sheets	179.6
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	84.9
Value	512.7	[1]
Amount Shown on Consolidated Balance Sheets	$ 512.7

[1] See Notes 2 and 4 of Notes to the Consolidated Financial Statements.

[2] The term "foreign" includes foreign governments, foreign political subdivisions, foreign public utilities and all other bonds of foreign issuers. Substantially all of the Company’s foreign securities are denominated in U.S. dollars.