Summary of Business Segment Information (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Commercial Lines Segment
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	$ 127	$ 130	$ 366	$ 386
Personal Lines Segment
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	262	306	821	923
American Reliable Insurance Company | Personal Lines Segment
Segment Reporting Information [Line Items]
Ceded premiums written	$ (1,427)	$ 7,328	$ (185)	$ 30,910
Quota share agreement percentage	100.00%	100.00%	100.00%	100.00%