Segment Information - Summary of Segment Activity (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			5
Number of operating segments | segment			4
Summarized financial information for these segments
Insurance premiums and contract charges earned	$ 235,353	$ 239,681	$ 697,049	$ 657,562
Net investment income	93,718	93,071	256,403	279,329
Net income (loss)	36,474	25,454	85,524	151,442
Assets	13,003,388		13,003,388		$ 12,478,704
Intersegment eliminations
Summarized financial information for these segments
Net investment income	(564)	(569)	(1,697)	(934)
Assets	(65,083)		(65,083)		(64,072)
Property and Casualty
Summarized financial information for these segments
Insurance premiums and contract charges earned	166,070	170,483	488,756	512,626
Property and Casualty | Operating segments
Summarized financial information for these segments
Net investment income	13,632	10,726	30,250	33,587
Net income (loss)	15,810	14,194	53,669	34,347
Assets	1,354,685		1,354,685		1,327,099
Supplemental
Summarized financial information for these segments
Insurance premiums and contract charges earned	32,480	32,921	98,772	32,921
Assets	828,760		828,760		747,602
Supplemental | Operating segments
Summarized financial information for these segments
Net investment income	4,321	3,691	11,876	3,691
Net income (loss)	10,546	6,943	30,562	6,943
Retirement
Summarized financial information for these segments
Insurance premiums and contract charges earned	7,389	6,624	21,486	22,133
Retirement | Operating segments
Summarized financial information for these segments
Net investment income	58,115	60,770	166,684	188,193
Net income (loss)	7,792	5,915	16,578	(6,979)
Assets	8,609,262		8,609,262		8,330,127
Life
Summarized financial information for these segments
Insurance premiums and contract charges earned	29,414	29,653	88,035	89,882
Life | Operating segments
Summarized financial information for these segments
Net investment income	18,244	18,453	49,432	54,829
Net income (loss)	4,306	5,101	6,869	13,617
Assets	2,108,067		2,108,067		1,964,993
Corporate and Other
Summarized financial information for these segments
Net investment income	(30)	0	(142)	(37)
Net income (loss)	(1,980)	$ (6,699)	(22,154)	$ 103,514
Assets	$ 167,697		$ 167,697		$ 172,955