Statutory Information and Restrictions - Reconciliations of Statutory Capital, Surplus and Net Income (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
Insurance [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 944,139				$ 912,336								$ 944,139	$ 912,336
Increase (decrease) due to:
Deferred policy acquisition costs	257,826				267,580								257,826	267,580
Difference in policyholder reserves	111,188				98,360								111,188	98,360
Goodwill	47,396				47,396								47,396	47,396
Investment fair value adjustments on fixed maturity securities	415,775				301,518								415,775	301,518
Difference in investment reserves	111,225				125,805								111,225	125,805
Federal income tax liability	(162,634)				(228,090)								(162,634)	(228,090)
Net funded status of pension and other postretirement benefit obligations	(16,789)				(18,250)								(16,789)	(18,250)
Non-admitted assets and other, net	28,870				22,888								28,870	22,888
Shareholders' equity of parent company and non-insurance subsidiaries	12,046				11,648								12,046	11,648
Parent company short-term and long-term debt	(247,469)				(247,209)								(247,469)	(247,209)
Total shareholders' equity	1,501,573				1,293,982				$ 1,264,661				1,501,573	1,293,982	$ 1,264,661
Statutory net income of insurance subsidiaries													82,587	74,574	87,619
Net loss of non-insurance companies													(4,496)	(5,135)	(4,474)
Interest expense													(11,836)	(11,808)	(13,122)
Debt retirement costs													0	0	(2,338)
Tax benefit of interest expense and other parent company current tax adjustments													5,654	5,637	6,829
Combined net income													71,909	63,268	74,514
Increase (decrease) due to:
Deferred policy acquisition costs													9,385	19,442	13,249
Policyholder benefits													30,609	14,919	14,065
Federal income tax (expense) benefit													84,198	(5,312)	(6,678)
Investment reserves													(20,966)	(1,320)	7,339
Other adjustments, net													(5,676)	(7,232)	(9,007)
Net income	$ 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