Condensed Financial Information - Statement of Cash Flows (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
Operating Activities:
Net Income	$ (33,368)	$ 4,625	$ 32,104	$ 30,878	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 34,239	$ 40,494	$ 82,726
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Other									42,718	936	27,791
Net Cash Provided by Operating Activities									342,338	265,343	229,425
Investing Activities:
Sales									34,617
Purchases									(126,746)
Net Cash Used in Investing Activities									(260,147)	(264,029)	(202,183)
Financing Activities:
Proceeds from Employee Stock Purchase Plan									2,008	2,076	1,840
Dividends Paid									(8,319)	(6,633)	(3,930)
Payment of Employee Tax Withholding on Stock Compensation									(8,456)	(13,887)	(5,198)
Net Cash Used in Financing Activities									(14,767)	(18,444)	(7,288)
Parent Company
Operating Activities:
Net Income									34,239	40,494	82,726
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									(55,163)	(50,197)	(90,121)
Dividends Received from Subsidiaries									97,000	19,000	5,000
Other									9,757	5,841	5,737
Net Cash Provided by Operating Activities									85,833	15,138	3,342
Investing Activities:
Net (Increase) Decrease in Short-Term Investments											89,576
Net Cash Used in Investing Activities											89,576
Financing Activities:
Capital Contribution to Subsidiary									(76,083)	(1)	(79,250)
Proceeds from Employee Stock Purchase Plan									2,036	1,846	1,840
Dividends Paid									(8,319)	(6,633)	(3,930)
Payment of Employee Tax Withholding on Stock Compensation									(6,288)	(13,778)	(5,358)
Net Cash Used in Financing Activities									(88,654)	(18,566)	(86,698)
Increase (Decrease) in Cash									(2,821)	(3,428)	6,220
Cash at Beginning of Year				$ 9,235				$ 12,663	9,235	12,663	6,443
Cash at End of Year	$ 6,414				$ 9,235				$ 6,414	$ 9,235	$ 12,663