Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Detail) $ in Thousands		Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Commercial Lines Segment | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 194,887
Commercial Lines Segment | Property Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		65,375	$ 65,529	$ 64,459
IBNR	[1]	$ 3,247
Cumulative Number of Reported Claims | Claim		6,388
Commercial Lines Segment | Property Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 65,880	64,693
IBNR	[1]	$ 5,817
Cumulative Number of Reported Claims | Claim		4,864
Commercial Lines Segment | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 63,632
IBNR	[1]	$ 11,855
Cumulative Number of Reported Claims | Claim		4,047
Commercial Lines Segment | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 889,577
Commercial Lines Segment | Casualty Insurance | Accident Year 2007
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		205,257	208,412	210,327	$ 215,716	$ 221,276	$ 219,709	$ 229,255	$ 240,974	$ 238,374	$ 222,751
IBNR	[1]	$ 11,119
Cumulative Number of Reported Claims | Claim		8,690
Commercial Lines Segment | Casualty Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 129,740	134,352	138,558	146,142	148,019	149,564	160,325	170,855	138,417
IBNR	[1]	$ 10,002
Cumulative Number of Reported Claims | Claim		6,065
Commercial Lines Segment | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 89,939	92,666	96,206	104,392	104,803	104,518	96,956	93,748
IBNR	[1]	$ 9,537
Cumulative Number of Reported Claims | Claim		3,774
Commercial Lines Segment | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 83,997	90,683	94,484	101,113	102,252	101,830	79,188
IBNR	[1]	$ 11,891
Cumulative Number of Reported Claims | Claim		3,287
Commercial Lines Segment | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 96,761	109,420	115,193	117,288	117,602	115,441
IBNR	[1]	$ 18,682
Cumulative Number of Reported Claims | Claim		3,478
Commercial Lines Segment | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 54,674	63,123	65,637	65,911	61,340
IBNR	[1]	$ 14,766
Cumulative Number of Reported Claims | Claim		2,140
Commercial Lines Segment | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 65,220	66,855	68,089	63,807
IBNR	[1]	$ 14,390
Cumulative Number of Reported Claims | Claim		2,278
Commercial Lines Segment | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 56,620	58,873	61,325
IBNR	[1]	$ 19,267
Cumulative Number of Reported Claims | Claim		2,089
Commercial Lines Segment | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 55,141	55,628
IBNR	[1]	$ 24,547
Cumulative Number of Reported Claims | Claim		1,715
Commercial Lines Segment | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 52,228
IBNR	[1]	$ 37,773
Cumulative Number of Reported Claims | Claim		1,247
Personal Lines Segment | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 282,527
Personal Lines Segment | Property Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		137,611	139,508
IBNR	[1]	$ 3,299
Cumulative Number of Reported Claims | Claim		16,182
Personal Lines Segment | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 144,916
IBNR	[1]	$ 14,179
Cumulative Number of Reported Claims | Claim		15,724
Personal Lines Segment | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 45,376
Personal Lines Segment | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		21,986	20,566
IBNR	[1]	$ 8,243
Cumulative Number of Reported Claims | Claim		1,057
Personal Lines Segment | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 23,390
IBNR	[1]	$ 16,465
Cumulative Number of Reported Claims | Claim		745
Reinsurance Operations | Property Insurance
Claims Development [Line Items]
Cumulative Number of Reported Claims | Claim		109,192
Reinsurance Operations | Property Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 14,562	14,562	14,562	14,562	14,091	15,041	13,486
IBNR	[1]	248
Reinsurance Operations | Property Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		24,912	24,994	25,994	26,916	28,547	30,963
IBNR	[1]	1,194
Reinsurance Operations | Property Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		8,497	8,579	9,279	10,578	10,388
IBNR	[1]	616
Reinsurance Operations | Property Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		6,698	8,197	9,948	15,153
IBNR	[1]	1,008
Reinsurance Operations | Property Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		14,139	18,861	21,787
IBNR	[1]	1,979
Reinsurance Operations | Property Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		16,738	19,877
IBNR	[1]	7,364
Reinsurance Operations | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		23,646
IBNR	[1]	17,739
Reinsurance Operations | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		176,560
Reinsurance Operations | Casualty Insurance | Accident Year 2007
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		3,944	3,894	4,931	4,931	4,705	6,352	6,351	6,359	6,396	$ 3,645
IBNR	[1]	529
Reinsurance Operations | Casualty Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		9,835	9,435	10,340	10,340	10,167	8,997	8,988	8,758	$ 8,906
IBNR	[1]	476
Reinsurance Operations | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		31,453	31,419	31,340	30,850	30,533	25,444	23,818	$ 20,706
IBNR	[1]	391
Reinsurance Operations | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		60,701	61,792	61,062	62,628	57,916	53,279	$ 41,831
IBNR	[1]	2,755
Reinsurance Operations | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		43,657	46,510	47,980	44,692	48,846	$ 45,726
IBNR	[1]	2,860
Reinsurance Operations | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		17,360	17,579	17,123	15,624	$ 15,865
IBNR	[1]	1,018
Reinsurance Operations | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		1,013	1,172	1,262	$ 1,224
IBNR	[1]	927
Reinsurance Operations | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		2,060	2,095	$ 1,988
IBNR	[1]	1,995
Reinsurance Operations | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		2,911	$ 2,908
IBNR	[1]	2,782
Reinsurance Operations | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		3,626
IBNR	[1]	$ 3,627

[1] Incurred-but-not-reported liabilities plus expected development on reported claims