Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2020 USD ($)
Summary of Investments - Other Than Investments in Related Parties
Cost	$ 98,143.4
Amount as shown in the Consolidated Statement of Financial Position	106,306.9
Fixed maturities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	70,546.8
Value	78,710.3
Amount as shown in the Consolidated Statement of Financial Position	78,710.3
Fixed maturities | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	532.1
Value	532.1
Amount as shown in the Consolidated Statement of Financial Position	532.1
U.S. government and agencies | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	1,893.1
Value	2,111.5
Amount as shown in the Consolidated Statement of Financial Position	2,111.5
States and political subdivisions | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	8,004.9
Value	9,167.8
Amount as shown in the Consolidated Statement of Financial Position	9,167.8
Non-U.S. governments | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	881.6
Value	1,073.7
Amount as shown in the Consolidated Statement of Financial Position	1,073.7
Public utilities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	5,375.6
Value	6,114.3
Amount as shown in the Consolidated Statement of Financial Position	6,114.3
Redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	19.3
Value	20.8
Amount as shown in the Consolidated Statement of Financial Position	20.8
All other corporate bonds | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	35,895.0
Value	41,219.7
Amount as shown in the Consolidated Statement of Financial Position	41,219.7
Residential mortgage-backed pass-through securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	2,857.6
Value	2,986.8
Amount as shown in the Consolidated Statement of Financial Position	2,986.8
Commercial mortgage-backed securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,741.2
Value	4,942.3
Amount as shown in the Consolidated Statement of Financial Position	4,942.3
Collateralized debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,045.9
Value	4,027.5
Amount as shown in the Consolidated Statement of Financial Position	4,027.5
Other debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	6,832.6
Value	7,045.9
Amount as shown in the Consolidated Statement of Financial Position	7,045.9
Equity securities
Summary of Investments - Other Than Investments in Related Parties
Cost	2,013.4
Value	2,013.4
Amount as shown in the Consolidated Statement of Financial Position	2,013.4
Common stocks: Banks, trust and insurance companies
Summary of Investments - Other Than Investments in Related Parties
Cost	483.4
Value	483.4
Amount as shown in the Consolidated Statement of Financial Position	483.4
Common stocks: Public Utilities
Summary of Investments - Other Than Investments in Related Parties
Cost	6.8
Value	6.8
Amount as shown in the Consolidated Statement of Financial Position	6.8
Common stocks: Industrial, miscellaneous and all other
Summary of Investments - Other Than Investments in Related Parties
Cost	874.6
Value	874.6
Amount as shown in the Consolidated Statement of Financial Position	874.6
Other corporate
Summary of Investments - Other Than Investments in Related Parties
Cost	499.7
Value	499.7
Amount as shown in the Consolidated Statement of Financial Position	499.7
Non-redeemable preferred stock
Summary of Investments - Other Than Investments in Related Parties
Cost	148.9
Value	148.9
Amount as shown in the Consolidated Statement of Financial Position	148.9
Mortgage loans
Summary of Investments - Other Than Investments in Related Parties
Cost	17,343.0
Amount as shown in the Consolidated Statement of Financial Position	17,343.0
Real estate acquired in satisfaction of debt
Summary of Investments - Other Than Investments in Related Parties
Cost	1.2
Amount as shown in the Consolidated Statement of Financial Position	1.2
Other real estate
Summary of Investments - Other Than Investments in Related Parties
Cost	1,796.1
Amount as shown in the Consolidated Statement of Financial Position	1,796.1
Policy loans
Summary of Investments - Other Than Investments in Related Parties
Cost	784.0
Amount as shown in the Consolidated Statement of Financial Position	784.0
Investments: Other investments
Summary of Investments - Other Than Investments in Related Parties
Cost	5,126.8
Amount as shown in the Consolidated Statement of Financial Position	$ 5,126.8