Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Unrealized gains (losses) on non-impaired securities	Unrealized gains (losses) on non-impaired securities Cumulative effect, adjustment	Unrealized losses on impaired held to maturity securities	Unrealized losses on impaired held to maturity securities Cumulative effect, adjustment	Unrealized losses on impaired available for sale securities	Foreign currency translation adjustments	Benefit plan liability adjustment	Retained earnings	Retained earnings Cumulative effect, adjustment
Total stockholders’ equity, beginning of period at Dec. 31, 2018		$ 36	$ 41,716		$ (30,286)	$ 0	$ (7)		$ (2)	$ 4,577	$ (11,297)	$ 1,896,040	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period, net of tax	$ 93,730				97,982
Amortization							6
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							(3)		0
Other-than-temporary impairments, non-credit, net of tax									0
Recoveries, net of tax									0
Change in translation adjustments during period										519
Amortization of net prior service cost and net loss, net of tax	(1,512)										(1,512)
Net earnings	93,883											93,883
Total stockholders’ equity, end of period at Sep. 30, 2019	2,091,652	36	41,716	$ 59,977	67,696		(4)		(2)	5,096	(12,809)	1,989,923
Total stockholders’ equity, beginning of period at Jun. 30, 2019		36	41,716		50,730		(5)	$ 0	(2)	5,044	(12,305)	1,969,934	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period, net of tax	15,586				16,966
Amortization							2
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							(1)		0
Other-than-temporary impairments, non-credit, net of tax									0
Recoveries, net of tax									0
Change in translation adjustments during period										52
Amortization of net prior service cost and net loss, net of tax	(504)										(504)
Net earnings	19,989											19,989
Total stockholders’ equity, end of period at Sep. 30, 2019	2,091,652	36	41,716	59,977	67,696		(4)		(2)	5,096	(12,809)	1,989,923
Total stockholders’ equity, beginning of period at Dec. 31, 2019	2,116,430	36	41,716		70,665	$ 0	(4)		(2)	5,101	(15,652)	2,014,570	(3,032)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period, net of tax	53,569				50,899
Amortization							4
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							(2)		0
Other-than-temporary impairments, non-credit, net of tax									0
Recoveries, net of tax									0
Change in translation adjustments during period										(46)
Amortization of net prior service cost and net loss, net of tax	(6,199)										(6,199)
Net earnings	57,176											57,176
Total stockholders’ equity, end of period at Sep. 30, 2020	2,215,230	36	41,716	104,764	121,564		(2)		(2)	5,055	(21,851)	2,068,714
Total stockholders’ equity, beginning of period at Jun. 30, 2020		36	41,716		97,909		(3)	$ 0	(2)	4,970	(19,785)	2,057,889	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares exercised under stock option plan		0	0
Change in unrealized gains (losses) during period, net of tax	24,357				23,655
Amortization							1
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							0		0
Other-than-temporary impairments, non-credit, net of tax									0
Recoveries, net of tax									0
Change in translation adjustments during period										85
Amortization of net prior service cost and net loss, net of tax	(2,066)										(2,066)
Net earnings	10,825											10,825
Total stockholders’ equity, end of period at Sep. 30, 2020	$ 2,215,230	$ 36	$ 41,716	$ 104,764	$ 121,564		$ (2)		$ (2)	$ 5,055	$ (21,851)	$ 2,068,714