Segment Information - Summarized Financial Information by Segment (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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summarized financial information for these segments
Insurance premiums and contract charges earned													$ 794,703	$ 759,146	$ 731,880
Net investment income													373,630	361,186	332,600
Net income (loss)	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	169,459	83,765	93,482
Assets	11,198,340				10,576,824				10,056,972				11,198,340	10,576,824	10,056,972
Intersegment eliminations
Summarized financial information for these segments
Net investment income													(815)	(855)	(891)
Assets	(42,482)				(36,786)				(37,208)				(42,482)	(36,786)	(37,208)
Property and Casualty
Summarized financial information for these segments
Net income (loss)													17,790	25,644	40,043
Property and Casualty | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned													648,263	620,514	595,958
Net investment income													36,178	38,998	33,461
Assets	1,217,394				1,110,958				1,098,415				1,217,394	1,110,958	1,098,415
Retirement
Summarized financial information for these segments
Net income (loss)													88,473	50,674	43,384
Retirement | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned													28,003	24,937	25,378
Net investment income													261,994	249,410	228,378
Assets	8,063,912				7,449,777				7,001,411				8,063,912	7,449,777	7,001,411
Life
Summarized financial information for these segments
Net income (loss)													77,595	16,559	14,982
Life | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned													118,437	113,695	110,544
Net investment income													76,195	73,567	71,614
Assets	1,815,732				1,912,771				1,862,719				1,815,732	1,912,771	1,862,719
Corporate and Other | Corporate and Other
Summarized financial information for these segments
Net investment income													78	66	38
Net income (loss)													(14,399)	(9,112)	(4,927)
Assets	$ 143,784				$ 140,104				$ 131,635				$ 143,784	$ 140,104	$ 131,635