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Fair Value Measurement, Recurring (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | $ 44,500.1 |
Derivative Asset | 21.5 | $ 19.5
Investments Net Asset Value | 413.3 |
Other Long-term Investments | 706.5 | 646.8
Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 44,500.1 | 45,457.8
Marketable Securities, Equity Securities | 72.4 | 11.7
Other Long-term Investments | 507.2 | 438.4
Assets, Fair Value Disclosure, Recurring | 45,007.3 | 45,896.2
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 69.0 | 68.1
Liabilities, Fair Value Disclosure, Recurring | 69.0 | 68.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 9,196.6 | 4,170.4
Marketable Securities, Equity Securities | 24.9 | 0.2
Other Long-term Investments | 24.9 | 0.2
Assets, Fair Value Disclosure, Recurring | 9,221.5 | 4,170.6
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Liabilities, Fair Value Disclosure, Recurring | 0.0 | 0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 33,941.4 | 39,906.8
Marketable Securities, Equity Securities | 46.4 | 10.4
Other Long-term Investments | 67.9 | 29.9
Assets, Fair Value Disclosure, Recurring | 34,009.3 | 39,936.7
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 51.4 | 52.2
Liabilities, Fair Value Disclosure, Recurring | 51.4 | 52.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,362.1 | 1,380.6
Marketable Securities, Equity Securities | 1.1 | 1.1
Other Long-term Investments | 1.1 | 1.1
Assets, Fair Value Disclosure, Recurring | 1,363.2 | 1,381.7
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 17.6 | 15.9
Liabilities, Fair Value Disclosure, Recurring | 17.6 | 15.9
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Marketable Securities, Equity Securities | 0.0 | 0.0
Other Long-term Investments | 413.3 | 407.2
Assets, Fair Value Disclosure, Recurring | 413.3 | 407.2
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Liabilities, Fair Value Disclosure, Recurring | 0.0 | 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 21.5 | 19.5
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 44.2 | 46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0.0 | 0.0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 21.5 | 19.5
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 44.2 | 46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0.0 | 0.0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Derivative Asset | 0.0 | 0.0
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 7.2 | 5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 7.2 | 5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Credit Default Swaps | Fair Value, Measurements, Recurring |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0.0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability |  | 0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 17.6 | 15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 17.6 | 15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Liabilities, Fair Value Disclosure [Abstract] |  |
Derivative Liability | 0.0 | 0.0
Equity Method Investments | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments Net Asset Value | 413.3 | 407.2
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments Net Asset Value | 0.0 | 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments Net Asset Value | 0.0 | 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments Net Asset Value | 0.0 | 0.0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments Net Asset Value | 413.3 | 407.2
United States Government and Government Agencies and Authorities |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,546.3 | 1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,546.3 | 1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 495.3 | 460.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,051.0 | 1,022.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
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 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
States, Municipalities, and Political Subdivisions |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 2,357.3 | 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 2,357.3 | 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 2,321.6 | 2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 35.7 | 0.0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Foreign Governments |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 873.7 | 863.9
Foreign Governments | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 873.7 | 863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 841.4 | 863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 32.3 | 0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Public Utilities |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 7,981.7 | 8,236.5
Public Utilities | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 7,981.7 | 8,236.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 574.1 | 154.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 7,123.4 | 7,874.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 284.2 | 207.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Mortgage/Asset-Backed Securities |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,842.8 | 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,842.8 | 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 1,842.3 | 1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.5 | 0.0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 29,857.0 | 30,522.4
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 8,127.2 | 3,556.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 20,742.6 | 25,816.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 987.2 | 1,150.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Redeemable Preferred Stocks |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 41.3 | 42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 41.3 | 42.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 0.0 | 0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 19.1 | 19.2
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | 22.2 | 22.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |  |
Assets, Fair Value Disclosure [Abstract] |  |
Fixed Maturity Securities | $ 0.0 | $ 0.0

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