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	$ 78,144				81,016
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										467
Amortization of net prior service cost and net loss, net of tax	(1,008)										(1,008)
Net earnings	73,894											73,894
Total stockholders’ equity, end of period at Jun. 30, 2019	2,055,148	36	41,716	$ 43,462	50,730		(5)		(2)	5,044	(12,305)	1,969,934
Total stockholders’ equity, beginning of period at Mar. 31, 2019		36	41,716		11,589		(6)	$ 0	(2)	5,130	(11,801)	1,936,238	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	35,401				39,141
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										(86)
Amortization of net prior service cost and net loss, net of tax	(504)										(504)
Net earnings	33,696											33,696
Total stockholders’ equity, end of period at Jun. 30, 2019	2,055,148	36	41,716	43,462	50,730		(5)		(2)	5,044	(12,305)	1,969,934
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	29,212				27,244
Amortization							3
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										(131)
Amortization of net prior service cost and net loss, net of tax	(4,133)										(4,133)
Net earnings	46,351											46,351
Total stockholders’ equity, end of period at Jun. 30, 2020	2,182,730	36	41,716	83,089	97,909		(3)		(2)	4,970	(19,785)	2,057,889
Total stockholders’ equity, beginning of period at Mar. 31, 2020		36	41,716		(8,089)		(3)	$ 0	(2)	5,062	(17,718)	2,009,476	$ 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	107,365				105,998
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							(1)		0
Other-than-temporary impairments, non-credit, net of tax									0
Recoveries, net of tax									0
Change in translation adjustments during period										(92)
Amortization of net prior service cost and net loss, net of tax	(2,067)										(2,067)
Net earnings	48,413											48,413
Total stockholders’ equity, end of period at Jun. 30, 2020	$ 2,182,730	$ 36	$ 41,716	$ 83,089	$ 97,909		$ (3)		$ (2)	$ 4,970	$ (19,785)	$ 2,057,889