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 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, beginning of period at Dec. 31, 2017		$ 36	$ 41,716		$ 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, net of tax	$ (51,238)				(53,771)
Amortization						7
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						(4)	(1)
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							0
Change in translation adjustments during period	1,227							1,227
Amortization of net prior service cost and net loss, net of tax	8,403								8,403
Net earnings	94,982									94,982
Total stockholders’ equity, end of period at Sep. 30, 2018	1,883,017	36	41,716	$ (34,272)	(24,521)	(7)	(2)	4,450	(14,192)	1,875,537
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle, net of tax					0
Total stockholders’ equity, beginning of period at Jun. 30, 2018		36	41,716		(22,169)	(8)	(1)	4,430	(16,993)	1,839,896
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	(1,416)				(2,352)
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)	(1)
Other-than-temporary impairments, non-credit, net of tax							0
Recoveries, net of tax							0
Change in translation adjustments during period	20							20
Amortization of net prior service cost and net loss, net of tax	2,801								2,801
Net earnings	35,641									35,641
Total stockholders’ equity, end of period at Sep. 30, 2018	1,883,017	36	41,716	(34,272)	(24,521)	(7)	(2)	4,450	(14,192)	1,875,537
Total stockholders’ equity, beginning of period at Dec. 31, 2018	1,900,777	36	41,716		(30,286)	(7)	(2)	4,577	(11,297)	1,896,040
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							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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle, net of tax					0
Total stockholders’ equity, beginning of period at Jun. 30, 2019		36	41,716		50,730	(5)	(2)	5,044	(12,305)	1,969,934
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							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