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	(36,065)				(38,860)
Amortization						77
Other-than-temporary impairments, non-credit, net of tax						90
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						(149)	(188)
Recoveries, net of tax							374
Change in translation adjustments during period	555							555
Amortization of net prior service cost and net gain, net of tax	1,127								1,127
Net earnings	68,405									68,405
Stockholder dividends										(1,273)
Total stockholders’ equity at Sep. 30, 2013	1,421,838	3,635	37,767	39,812	53,112	(1,408)	(10)	3,144	(15,026)	1,340,624
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	14,101				10,772
Amortization						80
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						(53)	(1)
Recoveries, net of tax							2
Change in translation adjustments during period	(636)							(636)
Amortization of net prior service cost and net gain, net of tax	(527)								(527)
Net earnings	77,475									77,475
Stockholder dividends										(1,272)
Total stockholders’ equity at Sep. 30, 2014	$ 1,534,138	$ 3,636	$ 38,116	$ 47,717	$ 57,465	$ (1,260)	$ (1)	$ 2,605	$ (11,092)	$ 1,444,669