Fair Value Measurements by Input Level (Detail) - USD ($)
$ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
$ 34,443.6
|
$ 33,056.6
|
| Equity Securities |
|
21.0
|
20.0
|
| Other Long-term Investments |
|
$ 1,628.8
|
$ 1,670.4
|
| Derivative Asset, Statement of Financial Position [Extensible Enumeration] |
|
Other Long-term Investments
|
Other Long-term Investments
|
| Derivative Assets |
|
$ 73.0
|
$ 69.7
|
| Assets, Fair Value Disclosure |
|
$ 35,962.7
|
$ 34,602.6
|
| Liabilities |
|
|
|
| Derivative Liability, Statement of Financial Position [Extensible Enumeration] |
|
Other Liabilities
|
Other Liabilities
|
| Derivative Liabilities |
|
$ 223.0
|
$ 269.7
|
| Liabilities, Fair Value Disclosure |
|
223.0
|
269.7
|
| Forwards |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Derivative Assets |
|
3.9
|
2.4
|
| Liabilities |
|
|
|
| Derivative Liabilities |
|
183.2
|
223.9
|
| Embedded Derivative in Modified Coinsurance Arrangement |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Derivative Assets |
|
15.1
|
17.8
|
| Interest Rate Swaps |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Derivative Assets |
|
54.0
|
49.5
|
| Liabilities |
|
|
|
| Derivative Liabilities |
|
39.8
|
45.8
|
| Fair Value, Measurements, Recurring |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Other Long-term Investments |
|
1,519.1
|
1,546.0
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
8,036.5
|
3,959.0
|
| Equity Securities |
|
0.0
|
0.0
|
| Other Long-term Investments |
|
0.0
|
0.0
|
| Assets, Fair Value Disclosure |
|
8,036.5
|
3,959.0
|
| 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 |
|
26,220.8
|
28,930.6
|
| Equity Securities |
|
0.1
|
0.2
|
| Other Long-term Investments |
|
58.0
|
52.1
|
| Derivative Assets |
|
57.9
|
51.9
|
| Assets, Fair Value Disclosure |
|
26,278.8
|
28,982.7
|
| Liabilities |
|
|
|
| Derivative Liabilities |
|
223.0
|
269.7
|
| Liabilities, Fair Value Disclosure |
|
223.0
|
269.7
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Forwards |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Derivative Assets |
|
3.9
|
2.4
|
| Liabilities |
|
|
|
| Derivative Liabilities |
|
183.2
|
223.9
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest Rate Swaps |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Derivative Assets |
|
54.0
|
49.5
|
| Liabilities |
|
|
|
| Derivative Liabilities |
|
39.8
|
45.8
|
| Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
186.3
|
167.0
|
| Equity Securities |
|
20.9
|
19.8
|
| Other Long-term Investments |
|
36.0
|
37.6
|
| Derivative Assets |
|
15.1
|
17.8
|
| Assets, Fair Value Disclosure |
|
222.3
|
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 |
|
15.1
|
17.8
|
| 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 |
|
1,425.1
|
1,456.3
|
| Assets, Fair Value Disclosure |
|
1,425.1
|
1,456.3
|
| Liabilities |
|
|
|
| Liabilities, Fair Value Disclosure |
|
0.0
|
0.0
|
| United States Government and Government Agencies and Authorities |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
626.7
|
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 |
|
77.5
|
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 |
|
549.2
|
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 |
|
2,958.5
|
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 |
|
2,958.5
|
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 |
|
864.8
|
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 |
|
856.9
|
870.3
|
| Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
7.9
|
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 |
|
5,040.8
|
4,933.9
|
| Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
566.4
|
223.7
|
| Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
4,474.4
|
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] |
1,479.5
|
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] |
1,376.0
|
1,076.2
|
| Mortgage/Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
[1] |
103.5
|
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 |
|
23,465.5
|
22,462.1
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
7,392.6
|
3,656.0
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
15,998.0
|
18,742.2
|
| All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |
|
|
|
| Assets, Fair Value Disclosure |
|
|
|
| Fixed Maturity Securities |
|
74.9
|
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 |
|
7.8
|
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 |
|
7.8
|
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 |
|
1,425.1
|
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 |
|
$ 1,425.1
|
$ 1,456.3
|
|
|