Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 62,314	$ 80,446	$ 79,706	$ 65,758	$ 32,053	$ 58,543	$ 58,527	$ 47,876	$ 288,224	$ 196,999	$ 210,366
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									13,368	14,831	15,095
Deferred income tax (benefit) expense									(1,358)	26,912	(2)
Other income tax expense									0	10,095	0
Realized losses (gains) and impairments on investments									3,591	(1,152)	(256)
Equity in losses (earnings) of limited partnerships									822	(2,801)	(7,025)
(Gain) loss on disposal of fixed assets									(3,047)	98	59
Net amortization of bond premium									5,601	7,038	7,436
Decrease in deferred compensation									(3,886)	(2,681)	(4,561)
Limited partnership distributions									7,173	5,128	17,837
Increase in receivables from affiliates									(30,804)	(39,788)	(30,485)
Decrease (increase) in accrued investment income									1,590	(516)	(846)
Decrease (increase) in federal income taxes recoverable									21,738	(24,640)	6,687
(Increase) decrease in prepaid pension									(47,335)	(27,265)	10,524
Increase in prepaid expenses and other assets									(727)	(7,636)	(4,674)
Increase in accounts payable and accrued expenses									11,039	17,183	11,144
Increase in commissions payable									13,449	17,565	15,017
(Decrease) increase in accrued agent bonuses									(19,066)	7,756	8,020
Increase in contract liability									3,213	0	0
Net cash provided by operating activities									$ 263,585	$ 197,126	$ 254,336