Statutory Information and Restrictions - Reconciliations of Statutory Capital, Surplus and Net Income (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 903,564				$ 944,139								$ 903,564	$ 944,139
Increase (decrease) due to:
Deferred policy acquisition costs	298,742				257,826								298,742	257,826
Difference in policyholder reserves	142,601				111,188								142,601	111,188
Goodwill	47,396				47,396								47,396	47,396
Investment fair value adjustments on fixed maturity securities	142,512				415,775								142,512	415,775
Difference in investment reserves	105,430				111,225								105,430	111,225
Federal income tax liability	(115,667)				(162,634)								(115,667)	(162,634)
Net funded status of benefit plans	(15,495)				(16,789)								(15,495)	(16,789)
Non-admitted assets and other, net	20,412				28,870								20,412	28,870
Shareholders' equity of parent company and non-insurance subsidiaries	8,795				12,046								8,795	12,046
Parent company short-term and long-term debt	(247,740)				(247,469)								(247,740)	(247,469)
Total shareholders' equity	1,290,550				1,501,573				$ 1,293,982				1,290,550	1,501,573	$ 1,293,982
Statutory net income of insurance subsidiaries													45,977	82,587	74,574
Net loss of non-insurance companies													(9,755)	(4,496)	(5,135)
Interest expense													(11,892)	(11,836)	(11,808)
Tax benefit of interest expense and other parent company current tax adjustments													121	5,654	5,637
Combined net income													24,451	71,909	63,268
Increase (decrease) due to:
Deferred policy acquisition costs													1,015	9,385	19,442
Policyholder benefits													26,318	30,609	14,919
Federal income tax (expense) benefit													3,020	84,198	(5,312)
Investment reserves													(31,529)	(20,966)	(1,320)
Other adjustments, net													(4,932)	(5,676)	(7,232)
Net income	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 18,343	$ 169,459	$ 83,765