Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital:	Accumulated other comprehensive income (loss)	Accumulated other comprehensive income (loss)	Unrealized losses on impaired held to maturity securities:	Unrealized losses on impaired available for sale securities:	Foreign currency translation adjustments:	Benefit plan liability adjustment	Retained earnings
Total stockholders’ equity at Dec. 31, 2015		$ 36	$ 41,716		$ 12,347	$ (240)	$ (1)	$ 2,825	$ (14,602)	$ 1,569,905
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period	$ 12,621				10,466
Amortization						72
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquisition costs						(35)	0
Recoveries, net of tax							0
Change in translation adjustments during period	(164)							(164)
Amortization of net prior service cost and net gain, net of tax	(116)								(116)
Net earnings	100,892									100,892
Stockholder dividends										(1,273)
Total stockholders’ equity at Dec. 31, 2016	1,721,828	36	41,716	$ 10,552	22,813	(203)	(1)	2,661	(14,718)	1,669,524
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period	8,288				4,885
Amortization						(242)
Other-than-temporary impairments, non-credit, net of tax						631
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquisition costs						(194)	0
Recoveries, net of tax							0
Change in translation adjustments during period	(9)							(9)
Amortization of net prior service cost and net gain, net of tax	(3,873)								(3,873)
Net earnings	110,421									110,421
Stockholder dividends										(1,273)
Total stockholders’ equity at Dec. 31, 2017	1,832,174	36	41,716	14,281	33,664	(10)	(1)	3,223	(22,595)	1,776,141
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period	(56,818)				(59,536)
Amortization						9
Other-than-temporary impairments, non-credit, net of tax						0
Additional credit loss on previously impaired securities						0
Change in shadow deferred policy acquisition costs						(6)	(1)
Recoveries, net of tax							0
Change in translation adjustments during period	1,354							1,354
Amortization of net prior service cost and net gain, net of tax	11,298								11,298
Net earnings	116,758									116,758
Stockholder dividends										(1,273)
Total stockholders’ equity at Dec. 31, 2018	$ 1,900,777	$ 36	$ 41,716	$ (37,015)	$ (30,286)	$ (7)	$ (2)	$ 4,577	$ (11,297)	$ 1,896,040