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 at Dec. 31, 2013		$ 902,212		$ 3,540	$ 333,776	$ (155,801)	$ 692,337	$ 28,360
Beginning balance (in shares) at Dec. 31, 2013				35,419,752		7,022,760
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	[1]	(23)		$ 16	(39)
Shares issued (in shares)	[1]			165,940
Share-based compensation		11,507			11,507
Ending balance at Dec. 31, 2014		1,027,224		$ 3,556	345,244	$ (155,801)	787,666	46,559
Ending balance (in shares) at Dec. 31, 2014				35,585,692		7,022,760
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	[1]	(4,175)		$ 30	(4,205)
Shares issued (in shares)	[1]			298,846
Share-based compensation		14,997			14,997
Ending balance at Dec. 31, 2015		$ 1,096,148	[2]	$ 3,586	356,036	$ (155,801)	868,723	23,604
Ending balance (in shares) at Dec. 31, 2015		35,884,538		35,884,538		7,022,760
Net income		$ 82,726					82,726
Dividends paid		(3,930)					(3,930)
Changes in comprehensive income:
Change in net unrealized gain (loss) on investments		(5,355)						(5,355)
Change in net non-credit other-than-temporary impairment losses		(50)						(50)
Change in foreign currency translation gain (loss)		(9,324)						(9,324)
Other comprehensive income (loss)		(14,729)						(14,729)
Shares issued	[1]	349		$ 26	323
Shares issued (in shares)	[1]			262,361
Share-based compensation		17,624			17,624
Ending balance at Dec. 31, 2016		$ 1,178,188	[2]	$ 3,612	$ 373,983	$ (155,801)	$ 947,519	$ 8,875
Ending balance (in shares) at Dec. 31, 2016		36,146,899		36,146,899		7,022,760

[1] Includes shares issued under the stock plan, to directors and ESPP

[2] We completed a two-for-one stock split on January 20, 2017. All per share data has been retroactively restated on a post-split basis.