Condensed Financial Information - Statement of Income (Detail) - 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Net Investment Income	$ 29,159	$ 25,651	$ 24,601	$ 23,702	$ 22,982	$ 22,598	$ 22,265	$ 21,448	$ 103,113	$ 89,293	$ 79,451
Total Revenues	360,621	376,976	361,360	344,200	362,021	326,472	317,383	308,603	1,443,157	1,314,479	1,197,533
Expenses:
Interest Expense	3,866	3,891	3,864	3,864	3,863	3,862	3,861	3,861	15,485	15,447	15,435
Other (Income) Expense	(2,934)	(1,724)	(2,628)	117	3,201	1,699	411	(1,068)	(7,169)	4,243	(8,701)
Total Expenses	393,110	372,813	321,572	307,087	298,605	371,315	289,910	279,843	1,394,582	1,239,673	1,080,024
Loss Before Income Tax Benefit	(32,489)	4,163	39,788	37,113	63,416	(44,843)	27,473	28,760	48,575	74,806	117,509
Income Tax Expense	879	(462)	7,684	6,235	36,555	(16,864)	6,971	7,650	14,336	34,312	34,783
Net Income	$ (33,368)	$ 4,625	$ 32,104	$ 30,878	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	34,239	40,494	82,726
Parent Company
Revenues:
Net Investment Income									18	7	53
Total Revenues									18	7	53
Expenses:
Interest Expense									15,460	15,447	15,435
Other (Income) Expense									2,563	(28)	22
Total Expenses									18,023	15,419	15,457
Loss Before Income Tax Benefit									(18,005)	(15,412)	(15,404)
Income Tax Expense									2,919	(5,709)	(8,009)
Loss Before Equity in Undistributed Net Income of Wholly Owned Subsidiaries									(20,924)	(9,703)	(7,395)
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									55,163	50,197	90,121
Net Income									$ 34,239	$ 40,494	$ 82,726