Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments - Other Than Investments in Related Parties
Cost	$ 79,274.4
Amount as shown in the Consolidated Statement of Financial Position	82,102.7
Fixed maturities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	56,562.1
Value	59,388.4
Amount as shown in the Consolidated Statement of Financial Position	59,388.4
Fixed maturities | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	566.0
Value	566.0
Amount as shown in the Consolidated Statement of Financial Position	566.0
U.S. government and agencies | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	1,314.5
Value	1,351.7
Amount as shown in the Consolidated Statement of Financial Position	1,351.7
States and political subdivisions | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	6,446.1
Value	6,801.6
Amount as shown in the Consolidated Statement of Financial Position	6,801.6
Non-U.S. governments | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	820.5
Value	901.5
Amount as shown in the Consolidated Statement of Financial Position	901.5
Public utilities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	4,750.3
Value	5,071.5
Amount as shown in the Consolidated Statement of Financial Position	5,071.5
Redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	33.8
Value	36.6
Amount as shown in the Consolidated Statement of Financial Position	36.6
All other corporate bonds | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	29,888.9
Value	31,925.0
Amount as shown in the Consolidated Statement of Financial Position	31,925.0
Residential mortgage-backed pass-through securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	2,493.8
Value	2,523.3
Amount as shown in the Consolidated Statement of Financial Position	2,523.3
Commercial mortgage-backed securities | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	3,734.0
Value	3,708.3
Amount as shown in the Consolidated Statement of Financial Position	3,708.3
Collateralized debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	1,372.1
Value	1,359.3
Amount as shown in the Consolidated Statement of Financial Position	1,359.3
Other debt obligations | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	5,708.1
Value	5,709.6
Amount as shown in the Consolidated Statement of Financial Position	5,709.6
Equity portfolios | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	94.0
Value	96.0
Amount as shown in the Consolidated Statement of Financial Position	96.0
Equity portfolios | Trading
Summary of Investments - Other Than Investments in Related Parties
Cost	1,770.6
Value	1,770.6
Amount as shown in the Consolidated Statement of Financial Position	1,770.6
Common stocks: Banks, trust and insurance companies | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	48.3
Value	44.0
Amount as shown in the Consolidated Statement of Financial Position	44.0
Common stocks: Industrial, miscellaneous and all other | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	0.9
Value	2.0
Amount as shown in the Consolidated Statement of Financial Position	2.0
Non-redeemable preferred stock | Available-for-sale
Summary of Investments - Other Than Investments in Related Parties
Cost	44.8
Value	50.0
Amount as shown in the Consolidated Statement of Financial Position	50.0
Mortgage loans
Summary of Investments - Other Than Investments in Related Parties
Cost	14,150.5
Amount as shown in the Consolidated Statement of Financial Position	14,150.5
Real estate acquired in satisfaction of debt
Summary of Investments - Other Than Investments in Related Parties
Cost	6.1
Amount as shown in the Consolidated Statement of Financial Position	6.1
Other real estate
Summary of Investments - Other Than Investments in Related Parties
Cost	1,730.6
Amount as shown in the Consolidated Statement of Financial Position	1,730.6
Policy loans
Summary of Investments - Other Than Investments in Related Parties
Cost	808.3
Amount as shown in the Consolidated Statement of Financial Position	808.3
Investments: Other investments
Summary of Investments - Other Than Investments in Related Parties
Cost	3,586.2
Amount as shown in the Consolidated Statement of Financial Position	$ 3,586.2