Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	$ 47,435.0	$ 43,011.7
Derivative Assets	34.9	30.9
Other Long-term Investments	752.3	702.9
Liabilities, Fair Value Disclosure	3,353.6	3,175.5
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities, Fair Value Disclosure	2,588.0	1,429.8
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities, Fair Value Disclosure	765.6	1,745.7
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Liabilities, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	47,435.0	43,011.7
Equity Securities	32.4	29.2
Other Long-term Investments	624.7	543.9
Assets, Fair Value Disclosure	48,059.7	43,555.6
Liabilities, Fair Value Disclosure	71.4	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	9,353.9	5,031.9
Equity Securities	0.0	0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	9,353.9	5,031.9
Liabilities, Fair Value Disclosure	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	37,457.0	36,346.8
Equity Securities	27.8	24.6
Other Long-term Investments	62.7	55.5
Assets, Fair Value Disclosure	37,519.7	36,402.3
Liabilities, Fair Value Disclosure	33.9	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	624.1	1,633.0
Equity Securities	4.6	4.6
Other Long-term Investments	4.6	4.6
Assets, Fair Value Disclosure	628.7	1,637.6
Liabilities, Fair Value Disclosure	37.5	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	557.4	483.8
Assets, Fair Value Disclosure	557.4	483.8
Liabilities, Fair Value Disclosure	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	1,847.5	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	1,847.5	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	210.8	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	1,636.7	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	2,948.7	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	2,948.7	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	2,935.6	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	13.1	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	1,051.8	983.7
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,051.8	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	1,019.9	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	31.9	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	7,815.7	7,412.8
Public Utilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	7,815.7	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	453.9	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	7,265.6	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	96.2	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	1,513.2	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	1,513.2	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	1,452.0	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	61.2	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	32,218.1	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	8,689.2	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	23,107.2	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	421.7	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	40.0	39.9
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	40.0	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	40.0	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	0.0	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
Private Equity Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	557.4	483.8
Private Equity Partnerships | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	557.4	483.8
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	0.0	0.0
Private Equity Partnerships | 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 Investments	557.4	483.8
Derivatives | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	34.9	30.9
Derivative Liabilities	71.4	69.1
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	0.0	0.0
Derivative Liabilities	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 Assets	34.9	30.9
Derivative Liabilities	33.9	38.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	0.0	0.0
Derivative Liabilities	37.5	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 Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	34.7	30.4
Derivative Liabilities	32.8	32.8
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	34.7	30.4
Derivative Liabilities	32.8	32.8
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts | 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 Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Credit Default Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	0.2	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 Assets	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 Assets	0.2	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 Assets	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 Assets	0.0	0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liabilities	1.1	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 Liabilities	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 Liabilities	1.1	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 Liabilities	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 Liabilities	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 Liabilities	37.5	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 Liabilities	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 Liabilities	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 Liabilities	37.5	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 Liabilities	$ 0.0	$ 0.0