Schedule II Condensed Financial Information of Registrant - Condensed Statements of Operations (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
Revenues
Net investment income													$ 373,630	$ 361,186	$ 332,600
Realized investment gains													(3,406)	4,123	12,713
Total revenues	$ 302,993	$ 289,817	$ 291,436	$ 287,304	$ 282,873	$ 291,176	$ 283,558	$ 271,303	$ 276,106	$ 265,753	$ 268,470	$ 270,119	1,171,550	1,128,910	1,080,448
Expenses
Interest expense													11,948	11,808	13,122
Debt retirement costs													0	0	2,338
Total expenses													1,082,863	1,014,678	950,996
Income tax benefit													(80,772)	30,467	35,970
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
Parent Company [Member]
Revenues
Net investment income													34	20	33
Realized investment gains													0	0	0
Total revenues													34	20	33
Expenses
Interest expense													11,835	11,808	13,122
Debt retirement costs													0	0	2,338
Other													5,101	5,631	5,153
Total expenses													16,936	17,439	20,613
Loss before income tax benefit and equity in net earnings of subsidiaries													(16,902)	(17,419)	(20,580)
Income tax benefit													(6,667)	(6,076)	(7,202)
Loss before equity in net earnings of subsidiaries													(10,235)	(11,343)	(13,378)
Equity in net earnings of subsidiaries													179,694	95,108	106,860
Net income													$ 169,459	$ 83,765	$ 93,482