Condensed Financial Information - Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ASSETS
Cash and investments	$ 2,422,254	$ 2,233,498
Goodwill and other intangible assets	7,093	6,869
Other assets	26,587	26,650
Total assets	4,007,670	3,670,007	3,531,459
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior notes	114,424	114,276
Accounts payable and other liabilities	48,015	21,999
Total liabilities	3,128,185	2,866,572
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,558,046 shares for 2012 and 17,467,615 shares for 2011	1,755	1,746
Additional paid-in capital	329,452	322,133
Treasury stock, at cost (3,511,380 shares for 2012 and 2011)	(155,801)	(155,801)
Retained earnings	628,871	565,109
Accumulated other comprehensive income:
Total stockholders' equity	879,485	803,435	829,354	801,519
Total liabilities and stockholders' equity	4,007,670	3,670,007
Parent Company
ASSETS
Cash and investments	15,026	8,315
Investments in subsidiaries	955,024	895,047
Goodwill and other intangible assets	2,534	2,534
Other assets	23,219	13,806
Total assets	995,803	919,702
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior notes	114,424	114,276
Accounts payable and other liabilities	552	649
Accrued interest payable	1,342	1,342
Total liabilities	116,318	116,267
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,558,046 shares for 2012 and 17,467,615 shares for 2011	1,755	1,746
Additional paid-in capital	329,452	322,133
Treasury stock, at cost (3,511,380 shares for 2012 and 2011)	(155,801)	(155,801)
Retained earnings	628,871	565,109
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	67,161	60,860
Foreign currency translation adjustment, net of tax	8,047	9,388
Total stockholders' equity	879,485	803,435
Total liabilities and stockholders' equity	$ 995,803	$ 919,702