Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2009	$ 801,519	$ 1,721	$ 304,505	$ (18,296)	$ 469,934	$ 43,655
Beginning balance at Dec. 31, 2009	16,846,000	17,212,814		366,330
Net income	69,578				69,578
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	(660)					(660)
Change in net non-credit other- than-temporary impairment losses	(2,824)					(2,824)
Change in foreign currency translation gain (loss)	290					290
Other comprehensive income (loss)	(3,194)					(3,194)
Shares issued under stock plan (in shares)		61,626
Shares issued under stock plan	7	7
Share-based compensation	13,424		8,083	5,341
Treasury stock acquired (in shares)	1,264,284			1,165,943
Treasury stock acquired	(51,980)			(51,980)
Ending Balance at Dec. 31, 2010	829,354	1,728	312,588	(64,935)	539,512	40,461
Ending balance at Dec. 31, 2010	15,742,000	17,274,440		1,532,273
Net income	25,597				25,597
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	29,469					29,469
Change in net non-credit other- than-temporary impairment losses	(83)					(83)
Change in foreign currency translation gain (loss)	401					401
Other comprehensive income (loss)	29,787					29,787
Shares issued under stock plan (in shares)		193,175
Shares issued under stock plan	18	18
Share-based compensation	9,545		9,545
Treasury stock acquired (in shares)	1,979,107			1,979,107
Treasury stock acquired	(90,866)			(90,866)
Ending Balance at Dec. 31, 2011	803,435	1,746	322,133	(155,801)	565,109	70,248
Ending balance at Dec. 31, 2011	13,956,000	17,467,615		3,511,380
Net income	63,762				63,762
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	5,196					5,196
Change in net non-credit other- than-temporary impairment losses	1,106					1,106
Change in foreign currency translation gain (loss)	(1,342)					(1,342)
Other comprehensive income (loss)	4,960					4,960
Shares issued under stock plan (in shares)		90,431
Shares issued under stock plan	(52)	9	(61)
Share-based compensation	7,380		7,380
Ending Balance at Dec. 31, 2012	$ 879,485	$ 1,755	$ 329,452	$ (155,801)	$ 628,871	$ 75,208
Ending balance at Dec. 31, 2012	14,047,000	17,558,046		3,511,380