Segment Information - Summarized Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summarized financial information for these segments
Insurance premiums and contract charges earned	$ 233,648	$ 235,353	$ 225,431	$ 236,265	$ 240,392	$ 239,681	$ 208,096	$ 209,785	$ 201,905	$ 206,820	$ 205,610	$ 202,998	$ 930,697	$ 897,954	$ 817,333
Net investment income													357,596	365,064	376,507
Net income	47,791	$ 36,474	$ 30,578	$ 18,472	33,001	$ 25,454	$ 93,822	$ 32,166	(20,257)	$ 12,528	$ 5,917	$ 20,155	133,315	184,443	18,343
Assets	13,471,813				12,478,704				11,031,896				13,471,813	12,478,704	11,031,896
Intersegment eliminations
Summarized financial information for these segments
Net investment income													(2,259)	(1,503)	(772)
Assets	(90,135)				(64,072)				(41,800)				(90,135)	(64,072)	(41,800)
Property and Casualty
Summarized financial information for these segments
Insurance premiums and contract charges earned													650,168
Property and Casualty | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned														683,454	665,734
Net investment income													42,556	41,740	40,104
Net income													76,516	54,359	(14,243)
Assets	1,324,923				1,327,099				1,236,362				1,324,923	1,327,099	1,236,362
Supplemental
Summarized financial information for these segments
Insurance premiums and contract charges earned													130,694	65,815
Net investment income													17,818	7,480
Net income													43,089	17,989
Assets	811,457				747,602								811,457	747,602
Retirement
Summarized financial information for these segments
Insurance premiums and contract charges earned													29,649
Retirement | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned														29,083	31,269
Net investment income													229,853	245,475	262,634
Net income													20,038	(4,867)	41,736
Assets	9,198,723				8,330,127				7,866,969				9,198,723	8,330,127	7,866,969
Life
Summarized financial information for these segments
Insurance premiums and contract charges earned													120,186
Life | Operating Segments
Summarized financial information for these segments
Insurance premiums and contract charges earned														119,602	120,330
Net investment income													69,799	71,957	74,399
Net income													10,461	17,574	18,754
Assets	2,044,503				1,964,993				1,821,351				2,044,503	1,964,993	1,821,351
Corporate and Other
Summarized financial information for these segments
Net investment income													(171)	(85)	142
Net income													(16,789)	99,388	(27,904)
Assets	$ 182,342				$ 172,955				$ 149,014				$ 182,342	$ 172,955	$ 149,014