Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Non-Auto Liability (Detail) $ in Thousands	Dec. 31, 2018 USD ($) claim
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 963,752
Non-Auto Liability
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	731,407
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	496,564
All outstanding liabilities before period one, net of reinsurance	21,715
Liabilities for claims and claim adjustment expenses, net of reinsurance	256,558
Non-Auto Liability | Accident Year 2009
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	83,773
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	75,857
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	70,905
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	72,267
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	72,490
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	72,077
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	71,003
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	71,517
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	69,099
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	69,696
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,632
Cumulative Number of Reported Claims | claim	13,275
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period One	$ 15,389
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	28,725
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	41,424
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	49,895
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	55,391
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	61,277
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	63,039
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	64,755
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	65,441
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	66,047
Non-Auto Liability | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	91,191
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	85,498
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	83,724
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	82,287
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	82,145
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	82,087
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	80,920
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	78,279
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	77,985
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 2,255
Cumulative Number of Reported Claims | claim	7,820
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Two	$ 16,473
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	31,819
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	46,746
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	57,354
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	65,557
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	69,091
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	70,369
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	71,509
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	72,261
Non-Auto Liability | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	86,409
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	76,038
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	75,390
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	74,372
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	73,647
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	71,423
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	68,248
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	67,979
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 2,480
Cumulative Number of Reported Claims | claim	5,656
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Three	$ 13,848
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	31,943
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	41,814
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	52,003
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	56,791
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	60,706
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	62,414
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	63,121
Non-Auto Liability | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	83,146
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	80,470
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	78,644
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	75,226
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	68,017
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	63,630
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	64,118
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 3,490
Cumulative Number of Reported Claims | claim	4,785
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Four	$ 13,862
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	27,574
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	38,826
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	49,585
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	55,194
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	57,863
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	59,528
Non-Auto Liability | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	74,183
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	75,815
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	70,772
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	67,841
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	65,096
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	64,564
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 4,085
Cumulative Number of Reported Claims | claim	4,435
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Five	$ 12,794
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	22,743
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	32,474
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	42,504
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	47,987
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	51,672
Non-Auto Liability | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	83,084
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	75,550
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	72,624
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	67,339
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	67,865
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 5,575
Cumulative Number of Reported Claims | claim	5,914
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Six	$ 11,201
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	26,587
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	36,220
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	45,206
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	51,853
Non-Auto Liability | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	83,897
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	78,968
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	76,724
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	67,548
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 10,626
Cumulative Number of Reported Claims | claim	5,344
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Seven	$ 11,979
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	23,488
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	37,059
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	46,285
Non-Auto Liability | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	86,935
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	83,179
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	73,764
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 19,334
Cumulative Number of Reported Claims | claim	4,065
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Eight	$ 12,733
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	24,633
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	35,502
Non-Auto Liability | Accident Year 2017
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	102,616
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	88,902
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 27,488
Cumulative Number of Reported Claims | claim	7,400
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Nine	$ 14,865
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	37,139
Non-Auto Liability | Accident Year 2018
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	88,986
Total of Incurred-but- Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 52,227
Cumulative Number of Reported Claims | claim	10,347
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance Period Ten	$ 13,156