Supplementary Data on Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 30,133	$ 49,562	$ 56,150	$ 38,833	$ 25,296	$ 46,900	$ 49,047	$ 46,262	$ 174,678	$ 167,505	$ 162,611
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									16,461	15,656	14,998
Deferred income tax benefit									(14,584)	(3,305)	(5,002)
Realized losses (gains) and impairments on investments									1,066	(952)	(557)
Equity in earnings of limited partnerships									(16,983)	(10,929)	(21,694)
Net amortization of bond premium									8,160	7,225	6,864
(Decrease) increase in deferred compensation									(1,526)	6,149	6,762
Limited partnership distributions									14,112	15,327	27,050
Increase in receivables from affiliates									(12,835)	(34,778)	(19,655)
Decrease (increase) in accrued investment income									47	(915)	(374)
(Increase) decrease in federal income taxes recoverable									(499)	(8,421)	2,812
Decrease in prepaid pension									20,307	557	17,842
Decrease (increase) in prepaid expenses and other assets									1,193	(1,747)	(5,831)
Increase in accounts payable and accrued expenses									3,633	1,753	4,910
Increase in commissions payable									5,624	20,596	10,508
Increase in accrued agent bonuses									18,524	12,292	16,764
Net cash provided by operating activities									$ 217,378	$ 186,013	$ 218,008