Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	$ 46,440.2	$ 43,011.7
Derivative Assets	27.4	30.9
Other Long-term Investments	735.0	702.9
Liabilities, Fair Value Disclosure	3,653.2	3,175.5
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets		0.0
Liabilities, Fair Value Disclosure	2,171.4	1,429.8
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets		30.9
Liabilities, Fair Value Disclosure	1,481.8	1,745.7
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets		0.0
Liabilities, Fair Value Disclosure	0.0	0.0
Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets		0.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	46,440.2	43,011.7
Equity Securities	32.2	29.2
Other Long-term Investments	599.9	543.9
Assets, Fair Value Disclosure	47,040.1	43,555.6
Liabilities, Fair Value Disclosure	62.6	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	10,954.9	5,031.9
Equity Securities	0.0	0.0
Other Long-term Investments	0.0	0.0
Assets, Fair Value Disclosure	10,954.9	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	34,250.2	36,346.8
Equity Securities	27.6	24.6
Other Long-term Investments	55.0	55.5
Assets, Fair Value Disclosure	34,305.2	36,402.3
Liabilities, Fair Value Disclosure	36.2	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	1,235.1	1,633.0
Equity Securities	4.6	4.6
Other Long-term Investments	4.6	4.6
Assets, Fair Value Disclosure	1,239.7	1,637.6
Liabilities, Fair Value Disclosure	26.4	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	540.3	483.8
Assets, Fair Value Disclosure	540.3	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	1,886.2	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	1,886.2	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	716.1	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	1,170.1	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	2,708.0	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	2,708.0	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	2,708.0	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	1,004.7	983.7
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	1,004.7	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	972.8	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	7,613.7	7,412.8
Public Utilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed Maturity Securities	7,613.7	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	780.9	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	6,643.7	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	189.1	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	1,566.4	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	1,566.4	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	1,566.4	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	31,621.2	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	9,437.2	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	21,169.9	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	1,014.1	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	20.7	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	19.3	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
Foreign Exchange Contracts | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets	27.4
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
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	27.4	30.4
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
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
Credit Default Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Assets		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
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
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
Private Equity Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	540.3	483.8
Private Equity Partnerships | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Alternative Investments	540.3	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	540.3	483.8
Derivatives | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liabilities	62.6	69.1
Derivatives | 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
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liabilities	36.2	38.0
Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liabilities	26.4	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 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		30.4
Derivative Liabilities	34.6	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
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 Liabilities	34.6	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
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
Derivative Liabilities	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		0.5
Interest Rate Swaps | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative Liabilities	1.6	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	1.6	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	26.4	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	26.4	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