Fair Value Measurements by Input Level (Detail) - USD ($) $ in Millions		Sep. 30, 2026	Dec. 31, 2025
Assets, Fair Value Disclosure
Fixed Maturity Securities		$ 32,994.5	$ 33,056.6
Equity Securities		30.8	20.0
Other Long-term Investments		$ 1,664.1	$ 1,670.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration]		Other Long-term Investments	Other Long-term Investments
Derivative Assets		$ 69.0	$ 69.7
Assets, Fair Value Disclosure		$ 34,542.4	$ 34,602.6
Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]		Other Liabilities	Other Liabilities
Derivative Liabilities		$ 421.5	$ 269.7
Liabilities, Fair Value Disclosure		421.5	269.7
Forwards
Assets, Fair Value Disclosure
Derivative Assets			2.4
Liabilities
Derivative Liabilities		0.4	223.9
Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		18.7	17.8
Interest Rate Swaps
Assets, Fair Value Disclosure
Derivative Assets		4.7
Forward Benchmark Interest Rate Lock
Liabilities
Derivative Liabilities		362.4
Total Return Swap
Liabilities
Derivative Liabilities		20.6
Receive Fixed/Pay Fixed
Assets, Fair Value Disclosure
Derivative Assets		44.6	49.5
Liabilities
Derivative Liabilities		38.1	45.8
Foreign Exchange Forward
Assets, Fair Value Disclosure
Derivative Assets		1.0
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure
Other Long-term Investments		1,547.9	1,546.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		8,047.6	3,959.0
Equity Securities		0.0	0.0
Other Long-term Investments		0.0	0.0
Assets, Fair Value Disclosure		8,047.6	3,959.0
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Interest Rate Swaps
Assets, Fair Value Disclosure
Derivative Assets		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		24,735.0	28,930.6
Equity Securities		6.8	0.2
Other Long-term Investments		57.1	52.1
Derivative Assets		50.3	51.9
Assets, Fair Value Disclosure		24,792.1	28,982.7
Liabilities
Derivative Liabilities		421.5	269.7
Liabilities, Fair Value Disclosure		421.5	269.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards
Assets, Fair Value Disclosure
Derivative Assets			2.4
Liabilities
Derivative Liabilities		0.4	223.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps
Assets, Fair Value Disclosure
Derivative Assets		4.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forward Benchmark Interest Rate Lock
Liabilities
Derivative Liabilities		362.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Total Return Swap
Liabilities
Derivative Liabilities		20.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Receive Fixed/Pay Fixed
Assets, Fair Value Disclosure
Derivative Assets		44.6	49.5
Liabilities
Derivative Liabilities		38.1	45.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Forward
Assets, Fair Value Disclosure
Derivative Assets		1.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		211.9	167.0
Equity Securities		24.0	19.8
Other Long-term Investments		42.7	37.6
Derivative Assets		18.7	17.8
Assets, Fair Value Disclosure		254.6	204.6
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement
Assets, Fair Value Disclosure
Derivative Assets		18.7	17.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest Rate Swaps
Assets, Fair Value Disclosure
Derivative Assets		0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Equity Securities		0.0	0.0
Other Long-term Investments		1,448.1	1,456.3
Assets, Fair Value Disclosure		1,448.1	1,456.3
Liabilities
Liabilities, Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share | Interest Rate Swaps
Assets, Fair Value Disclosure
Derivative Assets		0.0
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure
Fixed Maturity Securities		794.6	545.0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		73.8	79.3
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		720.8	465.7
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
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
Fixed Maturity Securities		0.0	0.0
States, Municipalities, and Political Subdivisions
Assets, Fair Value Disclosure
Fixed Maturity Securities		2,706.4	3,058.1
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
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
Fixed Maturity Securities		2,706.4	3,058.1
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
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
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Foreign Governments
Assets, Fair Value Disclosure
Fixed Maturity Securities		835.2	870.3
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		827.8	870.3
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		7.4	0.0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Public Utilities
Assets, Fair Value Disclosure
Fixed Maturity Securities		4,836.0	4,933.9
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		570.9	223.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		4,265.1	4,710.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Mortgage/Asset-backed Securities
Assets, Fair Value Disclosure
Fixed Maturity Securities	[1],[2]	1,646.6	1,179.3
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities	[1]	0.0	0.0
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities	[1]	1,537.0	1,076.2
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities	[1]	109.6	103.1
Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities	[1]	0.0	0.0
All Other Corporate Bonds
Assets, Fair Value Disclosure
Fixed Maturity Securities		22,168.0	22,462.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		7,402.9	3,656.0
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		14,670.2	18,742.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		94.9	63.9
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Redeemable Preferred Stocks
Assets, Fair Value Disclosure
Fixed Maturity Securities		7.7	7.9
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Fixed Maturity Securities		7.7	7.9
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Fixed Maturity Securities		0.0	0.0
Private Equity Partnerships
Assets, Fair Value Disclosure
Alternative Investments		1,448.1	1,456.3
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure
Alternative Investments		0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure
Alternative Investments		0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure
Alternative Investments		0.0	0.0
Private Equity Partnerships | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure
Alternative Investments		$ 1,448.1	$ 1,456.3

[1] 1Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types.

[2] Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types