Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 293,304	$ 316,821	$ 288,224
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	21,195	16,813	13,368
Deferred income tax (benefit) expense	(5,162)	3,349	(1,358)
Lease amortization expense	13,108	13,959	0
Realized (gains) losses and impairments on investments	(3,114)	(5,908)	3,591
(Gain) loss on disposal of fixed assets	(15)	75	(3,047)
Net investment income	5,878	543	6,423
Increase (decrease) in deferred compensation	7,611	(1,541)	(3,886)
Increase in receivables from affiliates	(26,548)	(19,505)	(30,804)
(Increase) decrease in accrued investment income	(713)	(170)	1,590
(Increase) decrease in federal income taxes recoverable	(2,202)	7,700	21,738
Decrease (increase) in prepaid pension	41,227	28,798	(47,335)
(Increase) decrease in prepaid expenses and other assets	(2,569)	(9,407)	6,446
(Decrease) increase in accounts payable and accrued expenses	(14,307)	(3,627)	11,039
(Decrease) increase in commissions payable	(625)	21,390	13,449
Increase (decrease) in accrued agent bonuses	14,105	(7,409)	(19,066)
Increase in contract liability	1,422	2,646	3,213
Net cash provided by operating activities	$ 342,595	$ 364,527	$ 263,585