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, 2011	$ 803,435	$ 1,746	$ 322,133	$ (155,801)	$ 565,109	$ 70,248
Beginning balance (in shares) at Dec. 31, 2011		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)
Total comprehensive income	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 (in shares) at Dec. 31, 2012		17,558,046		3,511,380
Net income	63,466				63,466
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	(44,094)					(44,094)
Change in net non-credit other- than-temporary impairment losses	320					320
Change in foreign currency translation gain (loss)	(3,074)					(3,074)
Total comprehensive income	(46,848)					(46,848)
Shares issued under stock plan (in shares)		151,830
Shares issued under stock plan	2,740	15	2,725
Share-based compensation	3,369		3,369
Ending balance at Dec. 31, 2013	902,212	1,770	335,546	(155,801)	692,337	28,360
Ending balance (in shares) at Dec. 31, 2013		17,709,876		3,511,380
Net income	95,329				95,329
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	23,096					23,096
Change in net non-credit other- than-temporary impairment losses	89					89
Change in foreign currency translation gain (loss)	(4,986)					(4,986)
Total comprehensive income	18,199					18,199
Shares issued under stock plan (in shares)		82,970
Shares issued under stock plan	(23)	8	(31)
Share-based compensation	11,507		11,507
Ending balance at Dec. 31, 2014	$ 1,027,224	$ 1,778	$ 347,022	$ (155,801)	$ 787,666	$ 46,559
Ending balance (in shares) at Dec. 31, 2014		17,792,846		3,511,380