Condensed Financial Information - Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ASSETS
Cash and investments	$ 2,233,498	$ 2,154,328
Goodwill and other intangible assets	6,869	6,925
Other assets	26,650	38,029
Total assets	3,670,007	3,531,459	3,453,994
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	114,276	114,138
Accounts payable and other liabilities	21,999	32,710
Total liabilities	2,866,572	2,702,105
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,467,615 shares for 2011 and 17,274,440 shares for 2010	1,746	1,728
Additional paid-in capital	322,133	312,588
Treasury stock, at cost (3,511,380 shares for 2011 and 1,532,273 shares for 2010)	(155,801)	(64,935)
Retained earnings	565,109	539,512
Accumulated other comprehensive income:
Total stockholders' equity	803,435	829,354	801,519
Total liabilities and stockholders' equity	3,670,007	3,531,459
Parent Company
ASSETS
Cash and investments	8,315	53,217
Investments in subsidiaries	895,047	877,999
Goodwill and other intangible assets	2,534	2,534
Other assets	13,806	12,028
Total assets	919,702	945,778
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	114,276	114,138
Accounts payable and other liabilities	649	946
Accrued interest payable	1,342	1,340
Total liabilities	116,267	116,424
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,467,615 shares for 2011 and 17,274,440 shares for 2010	1,746	1,728
Additional paid-in capital	322,133	312,588
Treasury stock, at cost (3,511,380 shares for 2011 and 1,532,273 shares for 2010)	(155,801)	(64,935)
Retained earnings	565,109	539,512
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	60,860	31,474
Foreign currency translation adjustment, net of tax	9,388	8,987
Total stockholders' equity	803,435	829,354
Total liabilities and stockholders' equity	$ 919,702	$ 945,778