Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net income	$ 32,053	$ 58,543	$ 58,527	$ 47,876	$ 45,786	$ 57,376	$ 61,309	$ 45,895	$ 196,999	$ 210,366	$ 174,678
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									14,929	15,154	16,461
Deferred income tax expense (benefit)									26,912	(2)	(14,584)
Other income tax expense									10,095
Realized (gains) losses and impairments on investments									(1,152)	(256)	1,066
Equity in earnings of limited partnerships									(2,801)	(7,025)	(16,983)
Net amortization of bond premium									7,038	7,436	8,160
Decrease in deferred compensation									(2,681)	(4,561)	(1,526)
Limited partnership distributions									5,128	17,837	14,112
Increase in receivables from affiliates									(39,788)	(30,485)	(12,835)
(Increase) decrease in accrued investment income									(516)	(846)	47
(Increase) decrease in federal income taxes recoverable									(24,640)	6,687	(499)
(Increase) decrease in prepaid pension									(27,265)	10,524	20,307
(Increase) decrease in prepaid expenses and other assets									(7,636)	(4,674)	1,193
Increase in accounts payable and accrued expenses									17,183	11,144	3,633
Increase in commissions payable									17,565	15,017	5,624
Increase in accrued agent bonuses									7,756	8,020	18,524
Net cash provided by operating activities									$ 197,126	$ 254,336	$ 217,378