Consolidated Statements of Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated other comprehensive income (loss) [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, 2011		$ 3,635	$ 37,767		$ 69,117	$ (2,321)	$ (608)	$ 2,368	$ (15,380)	$ 1,182,207
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period	27,211				22,855
Amortization						393
Other-than-temporary impairments, non-credit, net of tax						289
Additional credit loss on previously impaired securities						411
Change in shadow deferred policy acquisition costs						(198)	(424)
Recoveries, net of tax							836
Change in translation adjustments during period	221							221
Amortization of net prior service cost and net gain, net of tax	(773)								(773)
Net earnings	92,558									92,558
Stockholder dividends										(1,273)
Total stockholders’ equity at Dec. 31, 2012	1,391,680	3,635	37,767	76,786	91,972	(1,426)	(196)	2,589	(16,153)	1,273,492
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period	(42,218)				(45,279)
Amortization						210
Other-than-temporary impairments, non-credit, net of tax						95
Additional credit loss on previously impaired securities						54
Change in shadow deferred policy acquisition costs						(220)	(196)
Recoveries, net of tax							390
Change in translation adjustments during period	652							652
Amortization of net prior service cost and net gain, net of tax	5,588								5,588
Net earnings	96,247									96,247
Stockholder dividends										(1,273)
Total stockholders’ equity at Dec. 31, 2013	1,447,948	3,635	37,767	38,080	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	12,536				7,536
Amortization						84
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						(59)	(1)
Recoveries, net of tax							2
Change in translation adjustments during period	(556)							(556)
Amortization of net prior service cost and net gain, net of tax	(3,300)								(3,300)
Net earnings	105,588									105,588
Stockholder dividends										(1,272)
Total stockholders’ equity at Dec. 31, 2014	$ 1,556,320	$ 3,636	$ 38,116	$ 41,786	$ 54,229	$ (1,262)	$ (1)	$ 2,685	$ (13,865)	$ 1,472,782