Condensed Financial Information - Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ASSETS
Cash and investments	$ 2,574,586	$ 2,422,254
Goodwill and other intangible assets	7,177	7,093
Other assets	35,039	26,587
Total assets	4,169,452	4,007,670	3,670,007
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	263,308	114,424
Accounts payable and other liabilities	69,379	48,015
Total liabilities	3,267,240	3,128,185
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,709,876 shares for 2013 and 17,558,046 shares for 2012	1,770	1,755
Additional paid-in capital	335,546	329,452
Treasury stock, at cost (3,511,380 shares for 2013 and 2012)	(155,801)	(155,801)
Retained earnings	692,337	628,871
Accumulated other comprehensive income:
Total stockholders' equity	902,212	879,485	803,435	829,354
Total liabilities and stockholders' equity	4,169,452	4,007,670
Parent Company
ASSETS
Cash and investments	100,676	15,026
Investments in subsidiaries	1,040,214	955,024
Goodwill and other intangible assets	2,534	2,534
Other assets	26,538	23,219
Total assets	1,169,962	995,803
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	263,308	114,424
Accounts payable and other liabilities	802	552
Accrued interest payable	3,640	1,342
Total liabilities	267,750	116,318
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,709,876 shares for 2013 and 17,558,046 shares for 2012	1,770	1,755
Additional paid-in capital	335,546	329,452
Treasury stock, at cost (3,511,380 shares for 2013 and 2012)	(155,801)	(155,801)
Retained earnings	692,337	628,871
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	23,387	67,161
Foreign currency translation adjustment, net of tax	4,973	8,047
Total stockholders' equity	902,212	879,485
Total liabilities and stockholders' equity	$ 1,169,962	$ 995,803