Segment Information (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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summarized financial information for these segments
Insurance premiums and contract charges earned													$ 759,146	$ 731,880	$ 715,760
Net investment income													361,186	332,600	329,815
Net income (loss)	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 30,068	$ 25,357	$ 20,452	$ 28,366	83,765	93,482	104,243
Assets	10,576,824				10,056,972				9,768,346				10,576,824	10,056,972	9,768,346
Intersegment Eliminations [Member]
Summarized financial information for these segments
Net investment income													(855)	(891)	(925)
Assets	(36,786)				(37,208)				(36,736)				(36,786)	(37,208)	(36,736)
Property and casualty [Member]
Summarized financial information for these segments
Insurance premiums and contract charges earned													620,514	595,958	581,828
Net investment income													38,998	33,461	36,790
Net income (loss)													25,644	40,043	46,907
Assets	1,110,958				1,098,415				1,107,962				1,110,958	1,098,415	1,107,962
Retirement [Member]
Summarized financial information for these segments
Insurance premiums and contract charges earned													24,937	25,378	25,540
Net investment income													249,410	228,378	222,071
Net income (loss)													50,674	43,384	45,336
Assets	7,449,777				7,001,411				6,683,473				7,449,777	7,001,411	6,683,473
Life [Member]
Summarized financial information for these segments
Insurance premiums and contract charges earned													113,695	110,544	108,392
Net investment income													73,567	71,614	71,865
Net income (loss)													16,559	14,982	17,503
Assets	1,912,771				1,862,719				1,858,150				1,912,771	1,862,719	1,858,150
Corporate and other [Member]
Summarized financial information for these segments
Net investment income													66	38	14
Net income (loss)													(9,112)	(4,927)	(5,503)
Assets	$ 140,104				$ 131,635				$ 155,497				$ 140,104	$ 131,635	$ 155,497