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, 2011		$ 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	7,752				6,713
Amortization						6
Other-than-temporary impairments, non-credit, net of tax						(51)
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquistion costs						21
Other-than-temporary impairments, non-credit, net of tax
Recoveries, net of tax							0
Change in translation adjustments during period	386							386
Amortization of net prior service cost and net gain, net of tax	223								223
Net earnings	19,741									19,741
Stockholder dividends										0
Total stockholders’ equity at Mar. 31, 2012	1,303,824	3,635	37,767	60,474	75,829	(2,344)	(608)	2,754	(15,157)	1,201,948
Total stockholders’ equity at Dec. 31, 2012	1,391,680	3,635	37,767		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 during period, net of tax	(1,125)				(2,483)
Amortization						0
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						25
Change in shadow deferred policy acquistion costs						(17)	(133)
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							263
Change in translation adjustments during period	590							590
Amortization of net prior service cost and net gain, net of tax	381								381
Net earnings	16,632									16,632
Stockholder dividends										0
Total stockholders’ equity at Mar. 31, 2013	$ 1,406,938	$ 3,635	$ 37,767		$ 89,489	$ (1,418)	$ (66)	$ 3,179	$ (15,772)	$ 1,290,124