Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2012		17,558,046		3,511,380
Beginning balance at Dec. 31, 2012	$ 879,485	$ 1,755	$ 329,452	$ (155,801)	$ 628,871	$ 75,208
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)
Other comprehensive income (loss)	(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 (in shares) at Dec. 31, 2013		17,709,876		3,511,380
Ending balance at Dec. 31, 2013	902,212	$ 1,770	335,546	$ (155,801)	692,337	28,360
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)
Other comprehensive income (loss)	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 (in shares) at Dec. 31, 2014	17,792,846	17,792,846		3,511,380
Ending balance at Dec. 31, 2014	$ 1,027,224	$ 1,778	347,022	$ (155,801)	787,666	46,559
Net income	81,057				81,057
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments	(22,389)					(22,389)
Change in net non-credit other-than-temporary impairment losses	56					56
Change in foreign currency translation gain (loss)	(622)					(622)
Other comprehensive income (loss)	(22,955)					(22,955)
Shares issued under stock plan (in shares)		149,423
Shares issued under stock plan	(4,175)	$ 15	(4,190)
Share-based compensation	$ 14,997		14,997
Ending balance (in shares) at Dec. 31, 2015	17,942,269	17,942,269		3,511,380
Ending balance at Dec. 31, 2015	$ 1,096,148	$ 1,793	$ 357,829	$ (155,801)	$ 868,723	$ 23,604