Schedule II Condensed Financial Information of Registrant - Condensed Statements of Operations (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
Revenues
Net investment income													$ 376,507	$ 373,630	$ 361,186
Realized investment gains													(12,543)	(3,406)	4,123
Total revenues	$ 278,535	$ 311,318	$ 306,257	$ 295,489	$ 302,993	$ 289,817	$ 291,436	$ 287,304	$ 282,873	$ 291,176	$ 283,558	$ 271,303	1,191,599	1,171,550	1,128,910
Expenses
Interest expense													13,001	11,948	11,808
Total expenses													1,172,062	1,082,863	1,014,678
Income tax benefit													1,194	(80,772)	30,467
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
Parent Company [Member]
Revenues
Net investment income													100	34	20
Realized investment gains													0	0	0
Total revenues													100	34	20
Expenses
Interest expense													11,892	11,835	11,808
Other													10,898	5,101	5,631
Total expenses													22,790	16,936	17,439
Loss before income tax benefit and equity in net earnings of subsidiaries													(22,690)	(16,902)	(17,419)
Income tax benefit													(4,723)	(6,667)	(6,076)
Loss before equity in net earnings of subsidiaries													(17,967)	(10,235)	(11,343)
Equity in net earnings of subsidiaries													36,310	179,694	95,108
Net income													$ 18,343	$ 169,459	$ 83,765