Condensed Financial Information - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Goodwill and Other Intangible Assets	$ 27,146	$ 6,596
Current Income Tax Receivable, Net	0	16,667
Other Assets	96,807	73,171
Total Assets	5,603,449	5,224,622
Liabilities:
Reserves for Losses and Loss Adjustment Expenses	2,743,973	2,515,145	$ 2,289,727
Unearned Premiums	1,102,735	987,681
Reinsurance Balances Payable	145,027	136,192
Senior Notes	264,052	263,885
Accounts Payable and Other Liabilities	151,622	95,754
Total Liabilities	4,416,599	3,998,657
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 36,805 shares for 2018 and 36,530 shares for 2017)	3,678	3,650
Additional Paid-In Capital	378,274	376,868
Treasury Stock, at cost (7,023 shares for 2018 and 2017)	(155,801)	(155,801)
Retained Earnings	1,012,220	981,380
Accumulated Other Comprehensive Income:
Total Stockholders' Equity	1,186,850	1,225,965	1,178,188	$ 1,096,148
Total Liabilities and Stockholders' Equity	5,603,449	5,224,622
Parent Company
ASSETS
Cash	6,414	9,235	$ 12,663	$ 6,443
Investments in Subsidiaries	1,391,620	1,423,843
Goodwill and Other Intangible Assets	2,534	2,534
Current Income Tax Receivable, Net	35,553	30,573
Other Assets	19,686	28,063
Total Assets	1,455,807	1,494,248
Liabilities:
Senior Notes	264,052	263,885
Accounts Payable and Other Liabilities	1,731	1,224
Accrued Interest Payable	3,174	3,174
Total Liabilities	268,957	268,283
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,805 shares for 2018 and 36,530 shares for 2017)	3,678	3,650
Additional Paid-In Capital	378,274	376,868
Treasury Stock, at cost (7,023 shares for 2018 and 2017)	(155,801)	(155,801)
Retained Earnings	1,012,220	981,380
Accumulated Other Comprehensive Income:
Net Unrealized Gains (Losses) on Securities Available-for-Sale, Net of Tax	(51,513)	19,874
Foreign Currency Translation Adjustment, Net of Tax	(8)	(6)
Total Stockholders' Equity	1,186,850	1,225,965
Total Liabilities and Stockholders' Equity	$ 1,455,807	$ 1,494,248