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, 2012		$ 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	(34,232)				(36,628)
Amortization						77
Other-than-temporary impairments, non-credit, net of tax						23
Additional credit loss on previously impaired securities						14
Other-than-temporary impairments, non-credit, net of tax							0
Change in shadow deferred policy acquisition costs						(77)	(123)
Recoveries, net of tax							244
Change in translation adjustments during period	593							593
Amortization of net prior service cost and net gain, net of tax	754								754
Net earnings	43,623									43,623
Stockholder dividends										0
Total stockholders’ equity at Jun. 30, 2013	1,400,180	3,635	37,767	41,663	55,344	(1,389)	(75)	3,182	(15,399)	1,317,115
Total stockholders’ equity at Dec. 31, 2013	1,447,948	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 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
Other-than-temporary impairments, non-credit, net of tax							0
Change in shadow deferred policy acquisition costs						(67)	(1)
Recoveries, net of tax							2
Change in translation adjustments during period	(633)							(633)
Amortization of net prior service cost and net gain, 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