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	901
Change in unrealized gains during period, net of tax	11,318				7,188
Cumulative effect of change in accounting principle						0
Amortization						163
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						184	622
Other-than-temporary impairments, non-credit, net of tax							(1,117)
Recoveries, net of tax							0
Change in translation adjustments during period	(155)							(155)
Amortization of net prior service cost and net gain, net of tax	(819)								819
Net earnings	51,265									51,265
Stockholder dividends										(1,271)
Total stockholders’ equity at Sep. 30, 2011	1,277,396	3,635	38,041	58,112	69,687	(2,366)	(495)	2,430	(11,144)	1,177,608
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	28,578				24,705
Cumulative effect of change in accounting principle						0
Amortization						21
Other-than-temporary impairments, non-credit, net of tax						316
Additional credit loss on previously impaired securities						360
Change in shadow deferred policy acquistion costs						(178)	(955)
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							570
Change in translation adjustments during period	307							307
Amortization of net prior service cost and net gain, net of tax	(429)								429
Net earnings	67,309									67,309
Stockholder dividends										(1,273)
Total stockholders’ equity at Sep. 30, 2012	$ 1,368,396	$ 3,635	$ 37,767	$ 78,751	$ 93,821	$ (1,801)	$ (993)	$ 2,675	$ (14,951)	$ 1,248,243