Summary of Business Segment Information (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Commercial Lines
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	$ 132	$ 258	$ 391	$ 787
Personal Lines
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	304	310	918	957
Personal Lines | American Reliable Insurance Company
Segment Reporting Information [Line Items]
Ceded premiums written	$ 7,328	$ 18,749	$ 30,910	$ 41,749
Quota share agreement percentage	100.00%	100.00%	100.00%	100.00%