Condensed Financial Information - Statement of Cash Flows (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
Operating Activities:
Net Income	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Other									936	27,791	(13,654)
Net Cash Provided by Operating Activities									265,343	229,425	233,646
Fixed Maturities, Available-for-Sale
Sales									225,962	446,666	376,522
Purchases									(1,008,537)	(962,419)	(648,059)
Equity Securities
Sales									207,838	54,798	96,405
Purchases									(62,887)	(92,329)	(215,405)
Net (Increase) Decrease in Short-Term Investments									17,877	73,541	(38,468)
Net Cash Used in Investing Activities									(247,706)	(211,432)	(249,855)
Financing Activities:
Proceeds from Employee Stock Purchase Plan									2,076	1,840	1,352
Proceeds of Stock Issued from Exercise of Stock Options									0	0	29
Dividends Paid									(6,633)	(3,930)	0
Payment of Employee Tax Withholding on Stock Compensation									(13,887)	(5,198)	(6,022)
Net Cash Used in Financing Activities									(18,444)	(7,288)	(4,641)
Change in Cash									2,441	(5,258)	(20,850)
Cash at Beginning of Year				64,643				69,901	64,643	69,901	90,751
Cash at End of Period	67,084				64,643				67,084	64,643	69,901
Parent Company
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									(50,197)	(90,121)	(90,862)
Dividends Received from Subsidiaries									19,000	5,000
Other									5,841	5,737	9,542
Net Cash Provided by Operating Activities									15,138	3,342	(263)
Fixed Maturities, Available-for-Sale
Sales											1,250
Equity Securities
Sales									0	0	0
Purchases									0	0	0
Net (Increase) Decrease in Short-Term Investments									(8)	87,942	3,918
Net Cash Used in Investing Activities									(8)	87,942	5,168
Financing Activities:
Capital Contribution to Subsidiary									(1)	(79,250)
Proceeds from Employee Stock Purchase Plan									1,846	1,840	1,352
Proceeds of Stock Issued from Exercise of Stock Options											29
Dividends Paid									(6,633)	(3,930)
Payment of Employee Tax Withholding on Stock Compensation									(13,778)	(5,358)	(6,169)
Net Cash Used in Financing Activities									(18,566)	(86,698)	(4,788)
Change in Cash									(3,436)	4,586	117
Cash at Beginning of Year				$ 11,003				$ 6,417	11,003	6,417	6,300
Cash at End of Period	$ 7,567				$ 11,003				$ 7,567	$ 11,003	$ 6,417