Summary of Business Segment Information (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commercial Lines Segment
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	$ 714	$ 756	$ 1,047
Personal Lines
Segment Reporting Information [Line Items]
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations	862	948	1,270
American Reliable Insurance Company | Personal Lines
Segment Reporting Information [Line Items]
Ceded premiums written	$ (1,338)	$ 35,334	$ 55,829
Quota share agreement percentage	100.00%	100.00%	100.00%