Condensed Financial Information - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Investments	$ 2,937,226	$ 2,729,735
Cash	69,901	90,751	$ 86,509	$ 45,336
Goodwill and other intangible assets	6,807	7,013
Other assets	67,643	56,719
Total assets	4,584,012	4,476,185
Liabilities:
Reserves for losses and loss adjustment expenses	2,202,644	2,159,634	2,045,071
Unearned premiums	820,676	766,167
Reinsurance balances payable	107,411	152,774
Senior notes	263,580	263,440
Accounts payable and other liabilities	92,058	105,345
Total liabilities	$ 3,487,864	$ 3,448,961
Stockholders' equity:
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,942,269 shares for 2015 and 17,792,846 shares for 2014	$ 1,793	$ 1,778
Additional paid-in capital	357,829	347,022
Treasury stock, at cost (3,511,380 shares for 2015 and 2014)	(155,801)	(155,801)
Retained earnings	868,723	787,666
Accumulated other comprehensive income:
Total stockholders' equity	1,096,148	1,027,224	902,212	879,485
Total liabilities and stockholders' equity	4,584,012	4,476,185
Parent Company
ASSETS
Investments	89,586	94,732
Cash	6,417	6,300	$ 6,143	$ 2,981
Investments in subsidiaries	1,232,012	1,163,822
Goodwill and other intangible assets	2,534	2,534
Other assets	33,647	27,531
Total assets	1,364,196	1,294,919
Liabilities:
Senior notes	263,580	263,440
Accounts payable and other liabilities	1,294	1,081
Accrued interest payable	3,174	3,174
Total liabilities	$ 268,048	$ 267,695
Stockholders' equity:
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,942,269 shares for 2015 and 17,792,846 shares for 2014	$ 1,793	$ 1,778
Additional paid-in capital	357,829	347,022
Treasury stock, at cost (3,511,380 shares for 2015 and 2014)	(155,801)	(155,801)
Retained earnings	868,723	787,666
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	23,611	46,573
Foreign currency translation adjustment, net of tax	(7)	(14)
Total stockholders' equity	1,096,148	1,027,224
Total liabilities and stockholders' equity	$ 1,364,196	$ 1,294,919