Condensed Financial Information - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Investments	$ 3,421,883	$ 3,130,523
Cash	67,084	64,643	$ 69,901	$ 90,751
Investments in Subsidiaries	1,720	1,960
Goodwill and Other Intangible Assets	6,596	6,451
Current Income Tax Receivable, Net	16,667	20,556
Other Assets	73,171	52,030
Total Assets	5,224,622	4,814,037
Liabilities:
Reserves for Losses and Loss Adjustment Expenses	2,515,145	2,289,727	2,202,644
Unearned Premiums	987,681	887,344
Reinsurance Balances Payable	136,192	108,980
Senior Notes	263,885	263,728
Accounts Payable and Other Liabilities	95,754	86,070
Total Liabilities	3,998,657	3,635,849
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)	0	0
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016)	3,650	3,612
Additional Paid-In Capital	376,868	373,983
Retained Earnings	981,380	947,519
Accumulated Other Comprehensive Income:
Total Stockholders' Equity	1,225,965	1,178,188	1,096,148	1,027,224
Total Liabilities and Stockholders' Equity	5,224,622	4,814,037
Parent Company
ASSETS
Investments	1,668	1,660
Cash	7,567	11,003	$ 6,417	$ 6,300
Investments in Subsidiaries	1,423,843	1,381,652
Goodwill and Other Intangible Assets	2,534	2,534
Current Income Tax Receivable, Net	30,573	13,160
Other Assets	28,063	36,428
Total Assets	1,494,248	1,446,437
Liabilities:
Senior Notes	263,885	263,728
Accounts Payable and Other Liabilities	1,224	1,347
Accrued Interest Payable	3,174	3,174
Total Liabilities	268,283	268,249
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016)	3,650	3,612
Additional Paid-In Capital	376,868	373,983
Treasury Stock, at cost (7,023 shares for 2017 and 2016)	(155,801)	(155,801)
Retained Earnings	981,380	947,519
Accumulated Other Comprehensive Income:
Net Unrealized Gains (Losses) on Securities Available-for-Sale, Net of Tax	19,874	8,882
Foreign Currency Translation Adjustment, Net of Tax	(6)	(7)
Total Stockholders' Equity	1,225,965	1,178,188
Total Liabilities and Stockholders' Equity	$ 1,494,248	$ 1,446,437