Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Net investment income	$ 7,166	$ 6,495	$ 6,845	$ 6,708	$ 6,763	$ 6,947	$ 6,649	$ 6,670	$ 27,214	$ 27,029	$ 27,018
Fee and other income									361	534	562
Total revenues	104,886	102,336	100,624	96,173	94,309	88,003	87,511	86,512	404,019	356,335	321,248
Expenses
Interest expense											566
Total expenses									330,270	297,131	284,105
Income tax expense (benefit)									20,083	15,567	7,790
Net income	16,865	13,479	12,773	10,549	17,443	9,699	7,644	8,851	53,666	43,637	29,353
Parent Company [Member]
Revenues
Net investment income									1,541	1,051	571
Fee and other income									5,368	5,453	4,765
Total revenues									6,909	6,504	5,336
Expenses
Other operating costs									6,909	6,504	4,770
Interest expense											566
Total expenses									6,909	6,504	5,336
Income (loss) before income taxes and equity in earnings of subsidiaries									0	0	0
Income tax expense (benefit)									(13)	6	(9)
Gain (loss) before equity in earnings of subsidiaries									13	(6)	9
Equity in net income of subsidiaries									53,653	43,643	29,344
Net income									$ 53,666	$ 43,637	$ 29,353