Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($)
$ in Millions |
3 Months Ended |
6 Months Ended |
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
| Embedded Derivative in Modified Coinsurance Arrangement |
|
|
|
|
|
| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation |
|
|
|
|
|
| Derivatives Measured with Unobservable Inputs, Beginning Balance |
|
$ 14.8
|
$ 9.6
|
$ 17.8
|
$ 11.5
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings |
|
0.3
|
1.0
|
(2.7)
|
(0.9)
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3 |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Derivatives Measured with Unobservable Inputs, Ending Balance |
|
15.1
|
10.6
|
15.1
|
10.6
|
| Fair Value, Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
0.3
|
1.0
|
(2.7)
|
(0.9)
|
| Foreign Governments |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
|
0.0
|
|
0.0
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
|
0.0
|
|
0.0
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
0.2
|
|
(0.3)
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
|
0.0
|
|
0.0
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
|
0.0
|
|
0.0
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
|
7.7
|
|
8.2
|
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
|
0.0
|
|
0.0
|
|
| Assets Measured with Unobservable Inputs, Ending Balance |
|
7.9
|
|
7.9
|
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
0.2
|
|
(0.3)
|
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
0.0
|
|
0.0
|
|
| Public Utilities |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
|
0.0
|
0.0
|
0.0
|
(1.5)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
0.7
|
0.0
|
0.4
|
1.7
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
|
(31.0)
|
(9.0)
|
(41.0)
|
(21.3)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
|
30.3
|
9.0
|
40.6
|
21.1
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Assets Measured with Unobservable Inputs, Ending Balance |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Mortgage/Asset-backed Securities |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
[1] |
197.7
|
77.2
|
103.1
|
73.5
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
[1] |
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
[1] |
0.2
|
0.9
|
1.2
|
0.3
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
[1] |
6.7
|
7.1
|
15.6
|
11.9
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
[1] |
(6.8)
|
(3.7)
|
(16.4)
|
(4.2)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
[1] |
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
[1] |
(94.3)
|
(0.1)
|
0.0
|
(0.1)
|
| Assets Measured with Unobservable Inputs, Ending Balance |
[1] |
103.5
|
81.4
|
103.5
|
81.4
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
[1] |
0.2
|
0.9
|
1.2
|
0.3
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
[1] |
0.0
|
0.0
|
0.0
|
0.0
|
| All Other Corporate Bonds |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
|
60.4
|
32.3
|
63.9
|
71.5
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
|
0.0
|
0.0
|
0.0
|
(6.2)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
(0.5)
|
(0.1)
|
(0.6)
|
(8.3)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
|
7.5
|
1.0
|
7.5
|
1.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
|
(140.1)
|
(29.3)
|
(179.1)
|
(99.6)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
|
153.4
|
16.0
|
230.6
|
99.7
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
|
(5.8)
|
0.0
|
(47.4)
|
(38.2)
|
| Assets Measured with Unobservable Inputs, Ending Balance |
|
74.9
|
19.9
|
74.9
|
19.9
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
(1.0)
|
(0.1)
|
(1.7)
|
(8.3)
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fixed Maturity Securities |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
|
258.1
|
109.5
|
167.0
|
145.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
|
0.0
|
0.0
|
0.0
|
(7.7)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
0.6
|
0.8
|
0.7
|
(6.3)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
|
14.2
|
8.1
|
23.1
|
12.9
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
|
(177.9)
|
(42.0)
|
(236.5)
|
(125.1)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
|
191.4
|
25.0
|
279.4
|
120.8
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
|
(100.1)
|
(0.1)
|
(47.4)
|
(38.3)
|
| Assets Measured with Unobservable Inputs, Ending Balance |
|
186.3
|
101.3
|
186.3
|
101.3
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
(0.6)
|
0.8
|
(0.8)
|
(8.0)
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Perpetual Preferred and Equity Securities |
|
|
|
|
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |
|
|
|
|
|
| Assets Measured with Unobservable Inputs, Beginning Balance |
|
19.8
|
26.8
|
19.8
|
24.4
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings |
|
0.0
|
6.9
|
0.0
|
7.6
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases |
|
1.1
|
3.0
|
1.1
|
4.7
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities |
|
0.0
|
(12.9)
|
0.0
|
(12.9)
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Assets Measured with Unobservable Inputs, Ending Balance |
|
20.9
|
23.8
|
20.9
|
23.8
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |
|
0.0
|
0.0
|
0.0
|
0.0
|
| Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) |
|
$ 0.0
|
$ 0.0
|
$ 0.0
|
$ 0.7
|
|
|