Supplementary Information about Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Claims Development [Line Items]
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	$ 543,440
Commercial Lines Segment | Property Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	129,659
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	44,055
Commercial Lines Segment | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	55,786	$ 51,967	$ 39,643
Commercial Lines Segment | Property Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	37,712	28,541
Commercial Lines Segment | Property Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	36,161
Commercial Lines Segment | Property Insurance | All outstanding liabilities before 2016, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	7,811
Commercial Lines Segment | Casualty Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	433,326
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	281,462
Commercial Lines Segment | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	21,966	14,027	4,135
Commercial Lines Segment | Casualty Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	12,711	4,914
Commercial Lines Segment | Casualty Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,297
Commercial Lines Segment | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	80,337	79,896	77,771	$ 75,181	$ 71,108	$ 65,721	$ 53,738	$ 37,653	$ 19,154	$ 5,564
Commercial Lines Segment | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	70,300	69,615	67,277	65,377	58,913	50,520	34,659	19,926	5,503
Commercial Lines Segment | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	77,675	74,839	72,087	64,722	56,562	41,282	21,325	5,451
Commercial Lines Segment | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	39,899	39,596	36,360	31,231	22,456	11,884	3,500
Commercial Lines Segment | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	50,964	46,272	38,438	29,510	17,881	6,400
Commercial Lines Segment | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	39,361	33,697	26,268	15,690	3,968
Commercial Lines Segment | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	35,816	25,147	14,584	3,336
Commercial Lines Segment | Casualty Insurance | All outstanding liabilities before 2009, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	67,760
Personal Lines | Property Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	265,690
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	41,409
Personal Lines | Property Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	138,522	114,360
Personal Lines | Property Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	127,168
Personal Lines | Property Insurance | All outstanding liabilities before 2017, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	4,144
Personal Lines | Casualty Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	32,101
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	48,498
Personal Lines | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	9,843	6,885	3,507
Personal Lines | Casualty Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,803	2,132
Personal Lines | Casualty Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,340
Personal Lines | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	16,115	12,926	8,757	3,439
Personal Lines | Casualty Insurance | All outstanding liabilities before 2015, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	4,378
Reinsurance Operations | Property Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	60,616
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	100,780
Reinsurance Operations | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	7,161	5,704	2,071
Reinsurance Operations | Property Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	20,609	2,152
Reinsurance Operations | Property Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	21
Reinsurance Operations | Property Insurance | Accident Year 2012
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	7,612	7,584	7,527	7,648	7,221	5,481	1,127
Reinsurance Operations | Property Insurance | Accident Year 2013
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	5,735	5,255	5,111	5,835	4,008	723
Reinsurance Operations | Property Insurance | Accident Year 2014
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	12,339	11,182	10,460	9,035	2,243
Reinsurance Operations | Property Insurance | Accident Year 2015
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	7,139	6,768	5,163	742
Reinsurance Operations | Property Insurance | All outstanding liabilities before 2012, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	1,281
Reinsurance Operations | Casualty Insurance
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	144,249
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	27,236
Reinsurance Operations | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	0	0	0
Reinsurance Operations | Casualty Insurance | Accident Year 2017
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2	0
Reinsurance Operations | Casualty Insurance | Accident Year 2018
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	0
Reinsurance Operations | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	31,210	31,128	31,019	30,141	15,104	13,652	12,597	11,064	9,759	$ 1,986
Reinsurance Operations | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	57,042	56,960	55,848	55,315	39,123	36,090	30,754	21,447	$ 10,185
Reinsurance Operations | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	40,303	40,079	39,815	38,907	36,020	28,495	20,072	$ 7,968
Reinsurance Operations | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	15,625	15,790	15,696	15,534	11,658	9,435	$ 5,312
Reinsurance Operations | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	65	65	65	62	50	$ 123
Reinsurance Operations | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1	1	50	47	$ 88
Reinsurance Operations | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1	$ 1	$ 128	$ 107
Reinsurance Operations | Casualty Insurance | All outstanding liabilities before 2009, net of reinsurance
Claims Development [Line Items]
All outstanding liabilities, net of reinsurance	$ 1,452