Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) $ in Millions	Dec. 31, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,957.6
Casualty Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	2,369.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	1,110.5
Other Casualty outstanding liabilities including unpaid loss and ALAE prior to 2014, net of reinsurance	54.2
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	1,313.5
Casualty Lines | Short-Duration Insurance Contracts, Accident Year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	202.2	$ 185.3	$ 158.7	$ 212.1	$ 198.4	$ 191.2	$ 194.1	$ 187.3	$ 186.8	$ 186.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	175.6	174.9	174.3	161.4	141.4	119.4	98.2	70.5	30.7	11.7
IBNR & Expected Development on Reported Claims	$ 14.9
Cumulative number of reported claims | claim	5,764
Casualty Lines | Short-Duration Insurance Contracts, Accident Year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 159.4	157.1	162.4	184.5	178.7	179.6	191.6	204.9	205.6
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	137.1	132.6	131.7	128.9	110.8	91.7	58.3	33.4	12.3
IBNR & Expected Development on Reported Claims	$ 16.1
Cumulative number of reported claims | claim	5,853
Casualty Lines | Short-Duration Insurance Contracts, Accident Year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 225.2	216.1	147.6	257.5	233.9	231.7	227.0	228.1
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	188.6	171.8	164.3	158.4	124.6	72.8	43.4	12.7
IBNR & Expected Development on Reported Claims	$ 30.1
Cumulative number of reported claims | claim	8,831
Casualty Lines | Short-Duration Insurance Contracts, Accident Year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 190.3	174.4	194.3	241.3	245.9	261.2	253.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	148.6	146.3	140.6	130.0	100.7	58.6	16.2
IBNR & Expected Development on Reported Claims	$ 30.9
Cumulative number of reported claims | claim	9,352
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2019
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 143.4	144.2	359.3	233.5	243.5	248.1
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	101.9	97.9	90.5	80.1	49.9	15.2
IBNR & Expected Development on Reported Claims	$ 21.6
Cumulative number of reported claims | claim	9,210
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2020
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 233.2	203.3	190.2	213.3	235.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	116.8	79.9	54.4	33.9	13.3
IBNR & Expected Development on Reported Claims	$ 62.7
Cumulative number of reported claims | claim	7,137
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2021
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 255.8	247.6	243.7	257.1
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	104.3	70.3	37.4	13.1
IBNR & Expected Development on Reported Claims	$ 103.8
Cumulative number of reported claims | claim	7,591
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2022
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 300.4	302.4	306.7
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	72.9	42.4	12.9
IBNR & Expected Development on Reported Claims	$ 181.6
Cumulative number of reported claims | claim	7,728
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2023
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 328.5	332.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	51.8	13.8
IBNR & Expected Development on Reported Claims	$ 238.1
Cumulative number of reported claims | claim	6,341
Casualty Lines | Short-Duration Insurance Contract, Accident Year 2024
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 331.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	12.9
IBNR & Expected Development on Reported Claims	$ 289.3
Cumulative number of reported claims | claim	4,989
Specialty Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 1,132.5
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	817.0
Other Casualty outstanding liabilities including unpaid loss and ALAE prior to 2014, net of reinsurance	(6.2)
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	309.3
Specialty Lines | Short-Duration Insurance Contracts, Accident Year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	45.4	45.0	45.0	47.5	47.4	47.8	47.5	45.9	50.8	49.2
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	46.1	45.4	45.3	45.9	45.8	44.6	42.6	36.9	31.7	19.4
IBNR & Expected Development on Reported Claims	$ (0.8)
Cumulative number of reported claims | claim	3,354
Specialty Lines | Short-Duration Insurance Contracts, Accident Year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 41.8	41.7	42.2	43.4	43.7	44.9	41.2	41.0	40.6
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	40.5	40.5	40.2	42.1	41.1	39.7	34.6	28.5	18.8
IBNR & Expected Development on Reported Claims	$ 1.2
Cumulative number of reported claims | claim	4,559
Specialty Lines | Short-Duration Insurance Contracts, Accident Year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 67.6	67.3	70.9	75.4	73.0	67.8	66.0	64.2
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	67.6	66.7	65.9	65.0	58.6	49.3	37.6	24.4
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative number of reported claims | claim	7,835
Specialty Lines | Short-Duration Insurance Contracts, Accident Year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 86.2	84.6	87.9	95.7	91.5	94.0	91.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	82.4	80.2	78.4	73.3	62.7	51.0	32.0
IBNR & Expected Development on Reported Claims	$ 3.7
Cumulative number of reported claims | claim	8,398
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2019
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 86.5	85.4	83.6	96.0	98.4	104.9
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	82.1	79.1	75.1	64.3	51.6	30.9
IBNR & Expected Development on Reported Claims	$ 3.0
Cumulative number of reported claims | claim	9,194
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2020
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 143.3	144.7	135.0	106.5	98.3
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	117.4	106.9	93.1	62.9	31.0
IBNR & Expected Development on Reported Claims	$ 12.9
Cumulative number of reported claims | claim	9,094
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2021
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 152.5	150.1	149.9	127.7
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	111.2	96.8	84.1	33.8
IBNR & Expected Development on Reported Claims	$ 28.4
Cumulative number of reported claims | claim	9,903
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2022
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 198.3	193.6	191.0
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	128.4	103.0	51.8
IBNR & Expected Development on Reported Claims	$ 47.7
Cumulative number of reported claims | claim	9,248
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2023
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 155.5	170.5
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	98.1	56.2
IBNR & Expected Development on Reported Claims	$ 35.8
Cumulative number of reported claims | claim	8,417
Specialty Lines | Short-Duration Insurance Contract, Accident Year 2024
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 155.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	43.2
IBNR & Expected Development on Reported Claims	$ 79.5
Cumulative number of reported claims | claim	6,487
Run-off Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 4,232.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	3,135.4
Other Casualty outstanding liabilities including unpaid loss and ALAE prior to 2014, net of reinsurance	168.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	1,266.1
Run-off Lines | Short-Duration Insurance Contracts, Accident Year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	390.8	391.7	393.1	420.4	412.4	408.8	397.0	399.4	403.5	402.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	388.8	385.2	386.0	385.9	325.6	306.4	272.0	211.3	146.3	$ 83.3
IBNR & Expected Development on Reported Claims	$ 0.7
Cumulative number of reported claims | claim	21,158
Run-off Lines | Short-Duration Insurance Contracts, Accident Year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 449.8	448.9	444.9	471.6	457.0	458.3	459.0	442.6	440.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	432.0	436.0	430.2	428.0	374.2	353.1	275.9	113.7	$ 94.1
IBNR & Expected Development on Reported Claims	$ 15.6
Cumulative number of reported claims | claim	19,828
Run-off Lines | Short-Duration Insurance Contracts, Accident Year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 528.8	533.0	539.8	575.7	546.9	523.9	493.1	525.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	523.5	504.9	508.2	492.3	432.7	364.8	250.9	$ 200.7
IBNR & Expected Development on Reported Claims	$ 6.8
Cumulative number of reported claims | claim	21,751
Run-off Lines | Short-Duration Insurance Contracts, Accident Year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 609.5	599.6	584.6	578.9	556.0	563.3	460.2
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	552.8	510.2	493.7	430.3	366.4	298.2	$ 104.0
IBNR & Expected Development on Reported Claims	$ 47.9
Cumulative number of reported claims | claim	23,784
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2019
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 449.9	425.3	414.1	537.8	497.7	429.2
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	380.1	356.0	331.4	305.1	264.4	$ 102.6
IBNR & Expected Development on Reported Claims	$ 55.6
Cumulative number of reported claims | claim	23,774
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2020
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 516.6	474.7	403.5	448.1	461.0
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	374.7	312.0	248.5	194.6	$ 186.0
IBNR & Expected Development on Reported Claims	$ 68.8
Cumulative number of reported claims | claim	22,592
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2021
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 417.1	374.6	342.1	396.8
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	248.2	177.2	125.6	$ 72.0
IBNR & Expected Development on Reported Claims	$ 115.1
Cumulative number of reported claims | claim	18,914
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2022
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 317.1	303.9	288.1
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	114.2	82.2	$ 30.0
IBNR & Expected Development on Reported Claims	$ 132.9
Cumulative number of reported claims | claim	17,526
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2023
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 314.8	281.6
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	93.4	$ 20.5
IBNR & Expected Development on Reported Claims	$ 189.5
Cumulative number of reported claims | claim	13,258
Run-off Lines | Short-Duration Insurance Contract, Accident Year 2024
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 238.4
Short-duration insurance contracts, cumulative paid claims and allocated claim adjustment expense, net	27.7
IBNR & Expected Development on Reported Claims	$ 152.1
Cumulative number of reported claims | claim	9,260