Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Homeowners (Detail) $ in Thousands	Dec. 31, 2018 USD ($) claim
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 963,752
Homeowners
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	1,639,544
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,566,672
All outstanding liabilities before period one, net of reinsurance	23
Liabilities for claims and claim adjustment expenses, net of reinsurance	72,895
Homeowners | Accident Year 2009
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	183,437
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	178,420
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	179,249
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	177,534
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	177,798
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	177,989
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	178,372
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	178,073
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	178,008
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	177,916
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 2
Cumulative Number of Reported Claims | claim	31,489
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	$ 142,781
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	170,372
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	173,985
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	175,220
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	176,588
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	176,985
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	177,428
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	177,615
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	177,670
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	177,689
Homeowners | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	206,606
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	200,318
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	198,111
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	198,029
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	197,443
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	197,675
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	197,465
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	197,067
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	196,639
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 72
Cumulative Number of Reported Claims | claim	37,069
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	$ 149,755
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	189,046
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	193,006
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	195,365
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	195,714
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	196,281
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	196,419
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	196,504
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	196,480
Homeowners | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	203,301
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	200,356
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	198,757
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	197,581
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	197,381
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	197,451
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	197,239
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	197,070
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 89
Cumulative Number of Reported Claims | claim	38,758
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	$ 160,625
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	190,946
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	194,237
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	195,327
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	196,575
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	196,628
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	196,717
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	196,757
Homeowners | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	181,284
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	177,664
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	175,523
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	175,509
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	175,178
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	175,032
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	174,611
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 168
Cumulative Number of Reported Claims | claim	30,990
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	$ 143,797
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	169,415
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	171,842
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	173,170
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	173,676
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	174,139
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	174,247
Homeowners | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	152,208
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	149,080
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	149,272
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	148,231
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	147,927
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	147,444
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 171
Cumulative Number of Reported Claims | claim	20,035
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	$ 115,605
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	140,309
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	145,152
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	146,650
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	146,920
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	147,145
Homeowners | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	132,651
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	131,634
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	130,287
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	131,546
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	130,895
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 195
Cumulative Number of Reported Claims | claim	18,153
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	$ 96,300
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	122,601
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	126,245
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	129,467
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	130,059
Homeowners | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	125,430
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	124,199
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	123,619
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	123,824
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 262
Cumulative Number of Reported Claims | claim	17,721
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	$ 86,617
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	114,696
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	119,331
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	122,585
Homeowners | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	147,264
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	145,373
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	144,376
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 607
Cumulative Number of Reported Claims | claim	21,460
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	$ 105,415
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	136,796
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	140,972
Homeowners | Accident Year 2017
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	164,284
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	172,274
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,578
Cumulative Number of Reported Claims | claim	23,405
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	$ 116,075
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	159,107
Homeowners | Accident Year 2018
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	174,495
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 15,491
Cumulative Number of Reported Claims | claim	21,355
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	$ 121,631