Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Summary of Investments - Other Than Investments in Related Parties
Cost	$ 63,423.9
Amount as shown in the Consolidated Statement of Financial Position	66,153.5
U.S. government and agencies | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	772.3
Value	805.1
Amount as shown in the Consolidated Statement of Financial Position	805.1
States and political subdivisions | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	2,670.0
Value	2,882.7
Amount as shown in the Consolidated Statement of Financial Position	2,882.7
Non-U.S. governments | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	917.6
Value	1,096.7
Amount as shown in the Consolidated Statement of Financial Position	1,096.7
Public utilities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,566.0
Value	4,325.3
Amount as shown in the Consolidated Statement of Financial Position	4,325.3
Redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	91.3
Value	89.0
Amount as shown in the Consolidated Statement of Financial Position	89.0
All other corporate bonds | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	27,296.8
Value	29,142.2
Amount as shown in the Consolidated Statement of Financial Position	29,142.2
Residential mortgage-backed pass-through securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	3,155.8
Value	3,343.0
Amount as shown in the Consolidated Statement of Financial Position	3,343.0
Commercial mortgage-backed securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	3,894.3
Value	3,413.7
Amount as shown in the Consolidated Statement of Financial Position	3,413.7
Collateralized debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	399.7
Value	338.8
Amount as shown in the Consolidated Statement of Financial Position	338.8
Other debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	3,606.9
Value	3,570.2
Amount as shown in the Consolidated Statement of Financial Position	3,570.2
Fixed maturities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	47,370.7
Value	49,006.7
Amount as shown in the Consolidated Statement of Financial Position	49,006.7
Fixed maturities | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	971.7
Value	971.7
Amount as shown in the Consolidated Statement of Financial Position	971.7
Common stocks: Industrial, miscellaneous and all other | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	5.9
Value	6.9
Amount as shown in the Consolidated Statement of Financial Position	6.9
Non-redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	69.0
Value	70.2
Amount as shown in the Consolidated Statement of Financial Position	70.2
Equity securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	74.9
Value	77.1
Amount as shown in the Consolidated Statement of Financial Position	77.1
Equity securities | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	404.8
Value	404.8
Amount as shown in the Consolidated Statement of Financial Position	404.8
Mortgage loans
Summary of Investments - Other Than Investments in Related Parties
Cost	10,829.3
Amount as shown in the Consolidated Statement of Financial Position	10,727.2
Real estate acquired in satisfaction of debt
Summary of Investments - Other Than Investments in Related Parties
Cost	230.0
Amount as shown in the Consolidated Statement of Financial Position	229.6
Other real estate
Summary of Investments - Other Than Investments in Related Parties
Cost	863.3
Amount as shown in the Consolidated Statement of Financial Position	863.3
Policy loans
Summary of Investments - Other Than Investments in Related Parties
Cost	885.1
Amount as shown in the Consolidated Statement of Financial Position	885.1
Investments: Other investments
Summary of Investments - Other Than Investments in Related Parties
Cost	1,794.1
Amount as shown in the Consolidated Statement of Financial Position	$ 2,988.0