Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 59,587	$ 94,169	$ 87,754	$ 75,311	$ 62,314	$ 80,446	$ 79,706	$ 65,758	$ 316,821	$ 288,224	$ 196,999
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									16,813	13,368	14,831
Deferred income tax expense (benefit)									3,349	(1,358)	26,912
Other income tax expense									0	0	10,095
Lease amortization expense									13,959	0	0
Realized (gains) losses and impairments on investments									(5,908)	3,591	(1,152)
Equity in (earnings) losses of limited partnerships									(660)	822	(2,801)
Loss (gain) on disposal of fixed assets									75	(3,047)	98
Net amortization of bond premium									1,203	5,601	7,038
Decrease in deferred compensation									(1,541)	(3,886)	(2,681)
Limited partnership distributions									1,931	7,173	5,128
Increase in receivables from affiliates									(19,505)	(30,804)	(39,788)
(Increase) decrease in accrued investment income									(170)	1,590	(516)
Decrease (increase) in federal income taxes recoverable									7,700	21,738	(24,640)
Decrease (increase) in prepaid pension									28,798	(47,335)	(27,265)
Increase in prepaid expenses and other assets									(11,338)	(727)	(7,636)
(Decrease) increase in accounts payable and accrued expenses									(3,627)	11,039	17,183
Increase in commissions payable									21,390	13,449	17,565
(Decrease) increase in accrued agent bonuses									(7,409)	(19,066)	7,756
Increase in contract liability									2,646	3,213	0
Net cash provided by operating activities									$ 364,527	$ 263,585	$ 197,126