Condensed Financial Information - Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
ASSETS
Investments		$ 3,130,523		$ 2,937,226
Cash		64,643		69,901		$ 90,751	$ 86,509
Investments in subsidiaries		1,960
Goodwill and other intangible assets		6,451		6,807
Other assets		52,030		67,643
Total assets		4,814,037		4,584,012
Liabilities:
Reserves for losses and loss adjustment expenses		2,289,727		2,202,644		2,159,634
Unearned premiums		887,344		820,676
Reinsurance balances payable		108,980		107,411
Senior notes		263,728		263,580
Accounts payable and other liabilities		86,070		93,553
Total liabilities		3,635,849		3,487,864
Stockholders' equity:
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued	[1]
Common stock, $.10 par value, authorized 50,000,000 shares, issued 36,146,899 shares for 2016 and 35,884,538 shares for 2015	[1]	3,612		3,586
Additional paid-in capital	[1]	373,983		356,036
Treasury stock, at cost (7,022,760 shares for 2016 and 2015)	[1]	(155,801)		(155,801)
Retained earnings	[1]	947,519		868,723
Accumulated other comprehensive income:
Total stockholders' equity		1,178,188	[1]	1,096,148	[1]	1,027,224	902,212
Total liabilities and stockholders' equity		4,814,037		4,584,012
Parent Company
ASSETS
Investments		1,660		89,586
Cash		11,003		6,417		$ 6,300	$ 6,143
Investments in subsidiaries		1,381,652		1,232,012
Goodwill and other intangible assets		2,534		2,534
Other assets		49,588		33,647
Total assets		1,446,437		1,364,196
Liabilities:
Senior notes		263,728		263,580
Accounts payable and other liabilities		1,347		1,294
Accrued interest payable		3,174		3,174
Total liabilities		268,249		268,048
Stockholders' equity:
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued	[2]
Common stock, $.10 par value, authorized 50,000,000 shares, issued 36,146,899 shares for 2016 and 35,884,538 shares for 2015	[2]	3,612		3,586
Additional paid-in capital	[2]	373,983		356,036
Treasury stock, at cost (7,022,760 shares for 2016 and 2015)	[2]	(155,801)		(155,801)
Retained earnings	[2]	947,519		868,723
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	[2]	8,882		23,611
Foreign currency translation adjustment, net of tax	[2]	(7)		(7)
Total stockholders' equity	[2]	1,178,188		1,096,148
Total liabilities and stockholders' equity		$ 1,446,437		$ 1,364,196

[1] We completed a two-for-one stock split on January 20, 2017. All per share data has been retroactively restated on a post-split basis.

[2] We completed a two-for-one stock split on January 20, 2017. All share and per share data has been retroactively restated on a post-split basis.