Fair Values of Financial Instruments Fair Value Measurements by Input Level (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	$ 44,782.6	$ 43,011.7
Derivative Asset	24.6	30.9
Other Long-term Investments	696.7	702.9
Total Financial Liabilities, Fair Value	3,167.0	3,175.5
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total Financial Liabilities, Fair Value	2,847.6	1,429.8
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total Financial Liabilities, Fair Value	319.4	1,745.7
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total Financial Liabilities, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	44,782.6	43,011.7
Equity Securities	32.3	29.2
Other Long-term Investments	551.6	543.9
Assets, Fair Value Disclosure	45,334.2	43,555.6
Total Financial Liabilities, Fair Value	61.3	69.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	10,140.6	5,031.9
Equity Securities	27.5	0.0
Other Long-term Investments	27.5	0.0
Assets, Fair Value Disclosure	10,168.1	5,031.9
Total Financial Liabilities, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	33,869.4	36,346.8
Equity Securities	0.2	24.6
Other Long-term Investments	24.8	55.5
Assets, Fair Value Disclosure	33,894.2	36,402.3
Total Financial Liabilities, Fair Value	35.7	38.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	772.6	1,633.0
Equity Securities	4.6	4.6
Other Long-term Investments	4.6	4.6
Assets, Fair Value Disclosure	777.2	1,637.6
Total Financial Liabilities, Fair Value	25.6	31.1
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Equity Securities	0.0	0.0
Other Long-term Investments	494.7	483.8
Assets, Fair Value Disclosure	494.7	483.8
Total Financial Liabilities, Fair Value	0.0	0.0
United States Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,845.8	1,814.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,845.8	1,814.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	466.9	513.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,378.9	1,301.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	2,540.4	2,424.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	2,540.4	2,424.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	2,540.4	2,424.2
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,007.7	983.7
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,007.7	983.7
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	976.1	952.3
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	31.6	31.4
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Public Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	7,454.5	7,412.8
Public Utilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	7,454.5	7,412.8
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	731.2	286.4
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	6,499.3	7,041.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	224.0	84.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,556.1	1,582.7
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,556.1	1,582.7
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,556.1	1,582.7
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	30,337.9	28,754.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	8,942.5	4,232.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	20,899.4	23,026.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	496.0	1,495.8
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	40.2	39.9
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	40.2	39.9
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	19.2	18.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	21.0	21.1
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	24.5	30.4
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	24.5	30.4
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.1	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.1	0.5
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Asset	0.0	0.0
Private Equity Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	494.7	483.8
Private Equity Funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	494.7	483.8
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	0.0	0.0
Private Equity Funds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investment	494.7	483.8
Derivatives | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	61.3	69.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	35.7	38.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	25.6	31.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	25.6	31.1
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	25.6	31.1
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	3.8	5.2
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	3.8	5.2
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	31.9	32.8
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	31.9	32.8
Currency Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	0.0	0.0
Currency Swap | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liability	$ 0.0	$ 0.0