Fair Value Measurements by Input Level (Detail) - USD ($)
$ in Millions |
Mar. 31, 2025 |
Dec. 31, 2024 |
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
$ 35,751.4
|
$ 35,629.9
|
| Equity Securities |
26.9
|
24.5
|
| Other Long-term Investments |
$ 1,689.8
|
$ 1,694.4
|
| Derivative Asset, Statement of Financial Position [Extensible Enumeration] |
Other Long-term Investments
|
Other Long-term Investments
|
| Derivative Assets |
$ 100.0
|
$ 90.9
|
| Assets, Fair Value Disclosure |
$ 37,313.6
|
$ 37,195.9
|
| Liabilities |
|
|
| Derivative Liability, Statement of Financial Position [Extensible Enumeration] |
Other Liabilities
|
Other Liabilities
|
| Derivative Liabilities |
$ 220.4
|
$ 255.7
|
| Liabilities, Fair Value Disclosure |
220.4
|
255.7
|
| Forwards |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
13.2
|
6.5
|
| Liabilities |
|
|
| Derivative Liabilities |
186.9
|
$ 223.2
|
| Foreign Exchange Contracts |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Asset, Statement of Financial Position [Extensible Enumeration] |
|
Other Long-term Investments
|
| Derivative Assets |
76.9
|
|
| Liabilities |
|
|
| Derivative Liabilities |
30.0
|
$ 32.5
|
| Total Return Swap |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
0.3
|
|
| Liabilities |
|
|
| Derivative Liabilities |
3.5
|
|
| Embedded Derivative in Modified Coinsurance Arrangement |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
9.6
|
11.5
|
| Fair Value, Measurements, Recurring |
|
|
| Assets, Fair Value Disclosure |
|
|
| Other Long-term Investments |
1,562.2
|
1,566.0
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
8,063.2
|
4,180.5
|
| Equity Securities |
0.0
|
0.0
|
| Other Long-term Investments |
0.0
|
0.0
|
| Assets, Fair Value Disclosure |
8,063.2
|
4,180.5
|
| Liabilities |
|
|
| Liabilities, Fair Value Disclosure |
0.0
|
0.0
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
27,578.7
|
31,304.4
|
| Equity Securities |
0.1
|
0.1
|
| Other Long-term Investments |
90.5
|
79.5
|
| Derivative Assets |
90.4
|
79.4
|
| Assets, Fair Value Disclosure |
27,669.2
|
31,383.9
|
| Liabilities |
|
|
| Derivative Liabilities |
220.4
|
255.7
|
| Liabilities, Fair Value Disclosure |
220.4
|
255.7
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
13.2
|
6.5
|
| Liabilities |
|
|
| Derivative Liabilities |
186.9
|
223.2
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign Exchange Contracts |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
76.9
|
72.9
|
| Liabilities |
|
|
| Derivative Liabilities |
30.0
|
32.5
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Total Return Swap |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
0.3
|
|
| Liabilities |
|
|
| Derivative Liabilities |
3.5
|
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
109.5
|
145.0
|
| Equity Securities |
26.8
|
24.4
|
| Other Long-term Investments |
36.4
|
35.9
|
| Derivative Assets |
9.6
|
11.5
|
| Assets, Fair Value Disclosure |
145.9
|
180.9
|
| Liabilities |
|
|
| Liabilities, Fair Value Disclosure |
0.0
|
0.0
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forwards |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
|
0.0
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Embedded Derivative in Modified Coinsurance Arrangement |
|
|
| Assets, Fair Value Disclosure |
|
|
| Derivative Assets |
9.6
|
11.5
|
| 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 |
1,435.3
|
1,450.6
|
| Assets, Fair Value Disclosure |
1,435.3
|
1,450.6
|
| Liabilities |
|
|
| Liabilities, Fair Value Disclosure |
0.0
|
0.0
|
| United States Government and Government Agencies and Authorities |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
533.7
|
530.5
|
| United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
78.8
|
77.9
|
| United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
454.9
|
452.6
|
| 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 |
3,257.8
|
3,291.4
|
| 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 |
3,257.8
|
3,291.4
|
| 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 |
810.3
|
768.1
|
| 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 |
810.3
|
768.1
|
| Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
0.0
|
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 |
5,287.2
|
5,292.9
|
| Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
501.8
|
174.5
|
| Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
4,785.4
|
5,118.4
|
| 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 |
960.8
|
917.2
|
| Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
0.0
|
0.0
|
| Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
883.6
|
843.7
|
| Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
77.2
|
73.5
|
| Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
0.0
|
0.0
|
| All Other Corporate Bonds |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
24,893.9
|
24,822.2
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
7,482.6
|
3,928.1
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
17,379.0
|
20,822.6
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
| Assets, Fair Value Disclosure |
|
|
| Fixed Maturity Securities |
32.3
|
71.5
|
| 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
|
7.6
|
| 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
|
7.6
|
| 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 |
1,435.3
|
1,450.6
|
| 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 |
$ 1,435.3
|
$ 1,450.6
|