Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Unrealized gains on non-impaired securities [Member]	Accumulated Other-than-Temporary Impairment [Member]	Unrealized losses on impaired held to maturity securities [Member]	Unrealized losses on impaired available for sale securities [Member]	Foreign currency translation adjustments [Member]	Benefit plan liability adjustment [Member]	Retained Earnings [Member]
Total stockholders’ equity at Dec. 31, 2013		$ 3,635	$ 37,767		$ 46,693	$ (1,287)		$ (2)	$ 3,241	$ (10,565)	$ 1,368,466
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		1	349
Change in unrealized gains (losses) during period, net of tax	$ 21,894				19,523
Amortization						117
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquisition costs						(67)		(1)
Other-than-temporary impairments, non-credit, net of tax								0
Recoveries, net of tax								2
Change in translation adjustments during period	(633)								(633)
Amortization of net prior service cost and net loss, net of tax	(351)									(351)
Net earnings	49,009										49,009
Stockholder dividends											0
Total stockholders’ equity at Jun. 30, 2014	1,515,897	3,636	38,116	$ 56,670	66,216		$ (1,237)	(1)	2,608	(10,916)	1,417,475
Total stockholders’ equity at Dec. 31, 2014	1,556,320	3,636	38,116		54,229	(1,262)		(1)	2,685	(13,865)	1,472,782
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period, net of tax	(11,830)				(13,850)
Amortization						172
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquisition costs						$ (79)		0
Other-than-temporary impairments, non-credit, net of tax								0
Recoveries, net of tax								0
Change in translation adjustments during period	(12)								(12)
Amortization of net prior service cost and net loss, net of tax	(607)									(607)
Net earnings	50,184										50,184
Stockholder dividends											0
Total stockholders’ equity at Jun. 30, 2015	$ 1,592,128	$ 3,636	$ 38,116	$ 27,410	$ 40,379		$ (1,169)	$ (1)	$ 2,673	$ (14,472)	$ 1,522,966