Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Commercial Multi-Peril (Detail) $ in Thousands	Dec. 31, 2018 USD ($) claim
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 963,752
Commercial Multi-Peril
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	343,850
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	253,063
All outstanding liabilities before period one, net of reinsurance	1,908
Liabilities for claims and claim adjustment expenses, net of reinsurance	92,695
Commercial Multi-Peril | Accident Year 2009
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	41,027
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	38,666
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	36,610
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	35,354
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	34,884
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	34,381
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	34,529
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	34,079
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	33,515
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	33,423
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 17
Cumulative Number of Reported Claims | claim	3,516
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	$ 11,101
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	17,248
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	21,660
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	25,779
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	30,272
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	32,150
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	32,623
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	32,842
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	32,977
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	33,349
Commercial Multi-Peril | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	41,116
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	37,736
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	40,243
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	37,520
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	35,914
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	37,839
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	37,215
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	36,367
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	35,923
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 191
Cumulative Number of Reported Claims | claim	3,589
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	$ 12,511
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	17,490
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	22,135
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	27,152
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	31,378
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	33,384
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	34,888
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	34,764
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	34,903
Commercial Multi-Peril | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	42,185
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	40,825
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	39,037
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	38,160
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	38,456
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	36,945
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	37,014
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	36,638
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 485
Cumulative Number of Reported Claims | claim	3,559
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	$ 13,092
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	18,390
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	22,616
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	28,291
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	30,458
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	32,692
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	34,177
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	34,782
Commercial Multi-Peril | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	35,169
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	28,548
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	26,805
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	23,258
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	23,385
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	23,090
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	22,481
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 606
Cumulative Number of Reported Claims | claim	2,715
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	$ 11,525
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	14,454
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	16,263
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	18,670
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	20,716
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	21,026
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	21,352
Commercial Multi-Peril | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	33,979
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	27,592
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	27,867
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	26,970
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	25,948
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	26,028
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,122
Cumulative Number of Reported Claims | claim	2,220
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	$ 9,374
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	12,723
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	15,426
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	18,406
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	20,816
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	21,718
Commercial Multi-Peril | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	36,852
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	31,220
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	34,911
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	33,962
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	36,132
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,370
Cumulative Number of Reported Claims | claim	2,303
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	$ 12,001
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	16,484
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	20,199
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	24,602
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	27,339
Commercial Multi-Peril | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	33,997
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	31,488
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	29,023
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	32,282
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 2,238
Cumulative Number of Reported Claims | claim	2,203
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	$ 9,820
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	12,956
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	16,402
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	21,680
Commercial Multi-Peril | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	38,115
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	33,475
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	33,080
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 7,460
Cumulative Number of Reported Claims | claim	4,575
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	$ 11,327
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	17,193
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	19,085
Commercial Multi-Peril | Accident Year 2017
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	42,411
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	37,079
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 9,740
Cumulative Number of Reported Claims | claim	6,403
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	$ 12,458
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	20,828
Commercial Multi-Peril | Accident Year 2018
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	50,784
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 23,785
Cumulative Number of Reported Claims | claim	7,422
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	$ 18,027