Condensed Financial Information - Statement of Income (Detail) - 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Net Investment Income	$ 22,982	$ 22,598	$ 22,265	$ 21,448	$ 20,107	$ 19,875	$ 19,875	$ 19,594	$ 89,293	$ 79,451	$ 68,718
Total Revenues	362,021	326,472	317,383	308,603	310,040	305,288	294,107	288,098	1,314,479	1,197,533	1,058,989
Expenses:
Interest Expense	3,863	3,862	3,861	3,861	3,860	3,859	3,858	3,858	15,447	15,435	15,424
Other (Income) Expense	3,201	1,699	411	(1,068)	(1,905)	183	(4,430)	(2,549)	4,243	(8,701)	491
Total Expenses	298,605	371,315	289,910	279,843	278,583	275,400	270,864	255,177	1,239,673	1,080,024	941,515
Income (Loss) Before Income Taxes	63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense	36,555	(16,864)	6,971	7,650	9,866	7,875	7,053	9,989	34,312	34,783	36,417
Net Income (Loss)	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	40,494	82,726	81,057
Parent Company
Revenues:
Net Investment Income									7	53	147
Total Revenues									7	53	147
Expenses:
Interest Expense									15,447	15,435	15,424
Other (Income) Expense									(28)	22
Total Expenses									15,419	15,457	15,424
Income (Loss) Before Income Taxes									(15,412)	(15,404)	(15,277)
Income Tax Expense									(5,709)	(8,009)	(5,472)
Income (Loss) Before Equity in Undistributed Net Income of Wholly Owned Subsidiaries									(9,703)	(7,395)	(9,805)
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									$ 50,197	$ 90,121	$ 90,862