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]	Accumulated Other-than-Temporary Impairment [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, 2013		$ 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		0	0
Change in unrealized gains during period, net of tax	11,002				10,578
Amortization						57
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						(35)		(1)
Recoveries, net of tax								2
Change in translation adjustments during period	(511)								(511)
Amortization of net prior service cost and net gain, net of tax	(176)									(176)
Net earnings	19,826										19,826
Stockholder dividends											0
Total stockholders’ equity at Mar. 31, 2014	1,477,688	3,635	37,767	47,994	57,271		(1,265)	(1)	2,730	(10,741)	1,388,292
Total stockholders’ equity at Dec. 31, 2014	1,556,320	3,636	38,116		54,229	(1,262)		(1)	2,685	(13,865)	1,472,782
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains during period, net of tax	9,372				9,207
Amortization						104
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						(43)		0
Recoveries, net of tax								0
Change in translation adjustments during period	41								41
Amortization of net prior service cost and net gain, net of tax	(303)									(303)
Net earnings	21,326										21,326
Stockholder dividends											0
Total stockholders’ equity at Mar. 31, 2015	$ 1,586,652	$ 3,636	$ 38,116	$ 50,792	$ 63,436		$ (1,201)	$ (1)	$ 2,726	$ (14,168)	$ 1,494,108