Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 45,786	$ 57,376	$ 61,309	$ 45,895	$ 30,133	$ 49,562	$ 56,150	$ 38,833	$ 210,366	$ 174,678	$ 167,505
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									15,154	16,461	15,656
Deferred income tax benefit									(2)	(14,584)	(3,305)
Realized (gains) losses and impairments on investments									(256)	1,066	(952)
Equity in earnings of limited partnerships									(7,025)	(16,983)	(10,929)
Net amortization of bond premium									7,436	8,160	7,225
(Decrease) increase in deferred compensation									(4,561)	(1,526)	6,149
Limited partnership distributions									17,837	14,112	15,327
Increase in receivables from affiliates									(30,485)	(12,835)	(34,778)
(Increase) decrease in accrued investment income									(846)	47	(915)
Decrease (increase) in federal income taxes recoverable									6,687	(499)	(8,421)
Decrease in prepaid pension									10,524	20,307	557
(Increase) decrease in prepaid expenses and other assets									(4,674)	1,193	(1,747)
Increase in accounts payable and accrued expenses									11,144	3,633	1,753
Increase in commissions payable									15,017	5,624	20,596
Increase in accrued agent bonuses									8,020	18,524	12,292
Net cash provided by operating activities									$ 254,336	$ 217,378	$ 186,013