Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2019 USD ($)
Summary of Investments - Other Than Investments in Related Parties
Cost	$ 91,547.7
Amount as shown in the Consolidated Statement of Financial Position	96,351.4
Fixed maturities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	65,302.5
Value	70,106.2
Amount as shown in the Consolidated Statement of Financial Position	70,106.2
Fixed maturities | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	675.9
Value	675.9
Amount as shown in the Consolidated Statement of Financial Position	675.9
U.S. government and agencies | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	1,627.0
Value	1,724.2
Amount as shown in the Consolidated Statement of Financial Position	1,724.2
States and political subdivisions | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	6,857.1
Value	7,490.0
Amount as shown in the Consolidated Statement of Financial Position	7,490.0
Non-U.S. governments | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	852.3
Value	996.2
Amount as shown in the Consolidated Statement of Financial Position	996.2
Public utilities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,543.2
Value	4,191.0
Amount as shown in the Consolidated Statement of Financial Position	4,191.0
Redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	33.8
Value	34.6
Amount as shown in the Consolidated Statement of Financial Position	34.6
All other corporate bonds | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	32,416.1
Value	36,421.8
Amount as shown in the Consolidated Statement of Financial Position	36,421.8
Residential mortgage-backed pass-through securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	2,913.9
Value	2,982.4
Amount as shown in the Consolidated Statement of Financial Position	2,982.4
Commercial mortgage-backed securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,746.6
Value	4,850.2
Amount as shown in the Consolidated Statement of Financial Position	4,850.2
Collateralized debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	3,226.7
Value	3,215.3
Amount as shown in the Consolidated Statement of Financial Position	3,215.3
Other debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	8,085.8
Value	8,200.5
Amount as shown in the Consolidated Statement of Financial Position	8,200.5
Equity securities
Summary of Investments - Other Than Investments in Related Parties
Cost	1,879.4
Value	1,879.4
Amount as shown in the Consolidated Statement of Financial Position	1,879.4
Common stocks: Banks, trust and insurance companies
Summary of Investments - Other Than Investments in Related Parties
Cost	456.5
Value	456.5
Amount as shown in the Consolidated Statement of Financial Position	456.5
Common stocks: Public Utilities
Summary of Investments - Other Than Investments in Related Parties
Cost	7.0
Value	7.0
Amount as shown in the Consolidated Statement of Financial Position	7.0
Common stocks: Industrial, miscellaneous and all other
Summary of Investments - Other Than Investments in Related Parties
Cost	785.6
Value	785.6
Amount as shown in the Consolidated Statement of Financial Position	785.6
Other corporate
Summary of Investments - Other Than Investments in Related Parties
Cost	480.7
Value	480.7
Amount as shown in the Consolidated Statement of Financial Position	480.7
Non-redeemable preferred stock
Summary of Investments - Other Than Investments in Related Parties
Cost	149.6
Value	149.6
Amount as shown in the Consolidated Statement of Financial Position	149.6
Mortgage loans
Summary of Investments - Other Than Investments in Related Parties
Cost	16,486.9
Amount as shown in the Consolidated Statement of Financial Position	16,486.9
Real estate acquired in satisfaction of debt
Summary of Investments - Other Than Investments in Related Parties
Cost	4.1
Amount as shown in the Consolidated Statement of Financial Position	4.1
Other real estate
Summary of Investments - Other Than Investments in Related Parties
Cost	1,710.7
Amount as shown in the Consolidated Statement of Financial Position	1,710.7
Policy loans
Summary of Investments - Other Than Investments in Related Parties
Cost	798.0
Amount as shown in the Consolidated Statement of Financial Position	798.0
Investments: Other investments
Summary of Investments - Other Than Investments in Related Parties
Cost	4,690.2
Amount as shown in the Consolidated Statement of Financial Position	$ 4,690.2