Condensed 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]	Unrealized gains on non-impaired securities [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, 2010		$ 3,629	$ 37,140		$ 62,499	$ (2,713)	$ 0	$ 2,585	$ (11,963)	$ 1,127,614
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		6	1,211
Change in unrealized gains during period, net of tax	5,775				3,703
Cumulative effect of change in accounting principal						0
Amortization						165
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquistion costs						185
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							0
Change in translation adjustments during period	(134)							(134)
Amortization of net prior service cost and net gain, net of tax	(582)								582
Net earnings	32,001									32,001
Total stockholders’ equity at Jun. 30, 2011	1,256,510	3,635	38,351	54,909	66,202	(2,363)	0	2,451	(11,381)	1,159,615
Total stockholders’ equity at Dec. 31, 2011	1,276,785	3,635	37,767		69,116	(2,320)	(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 during period, net of tax	13,376				10,679
Cumulative effect of change in accounting principal						0
Amortization						15
Other-than-temporary impairments, non-credit, net of tax						92
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquistion costs						30	0
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							0
Change in translation adjustments during period	285							285
Amortization of net prior service cost and net gain, net of tax	(448)								448
Net earnings	42,987									42,987
Stockholder dividends										0
Total stockholders’ equity at Jun. 30, 2012	$ 1,331,321	$ 3,635	$ 37,767	$ 64,725	$ 79,795	$ (2,183)	$ (608)	$ 2,653	$ (14,932)	$ 1,225,194