Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Net investment income	$ 7,256	$ 6,923	$ 6,890	$ 6,833	$ 7,166	$ 6,495	$ 6,845	$ 6,708	$ 27,902	$ 27,214	$ 27,029
Fee and other income									316	361	536
Total revenues	101,760	97,470	99,936	101,788	104,886	102,336	100,624	96,173	400,954	404,019	356,335
Expenses
Total expenses									299,987	330,270	297,131
Income tax expense (benefit)									30,505	20,083	15,567
Net income	$ 23,073	$ 17,940	$ 14,319	$ 15,130	$ 16,865	$ 13,479	$ 12,773	$ 10,549	70,462	53,666	43,637
Parent Company [Member]
Revenues
Net investment income									854	1,541	1,051
Fee and other income									5,893	5,368	5,453
Total revenues									6,747	6,909	6,504
Expenses
Other operating costs									6,747	6,909	6,504
Total expenses									6,747	6,909	6,504
Income (loss) before income taxes and equity in earnings of subsidiaries									0	0	0
Income tax expense (benefit)									(57)	(13)	6
Gain (loss) before equity in earnings of subsidiaries									57	13	(6)
Equity in net income of subsidiaries									70,405	53,653	43,643
Net income									$ 70,462	$ 53,666	$ 43,637