Components of Current and Deferred Income Tax Expense (Benefit) (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
Current Income Tax Expense (Benefit):
Federal and Foreign									$ 27,768	$ 42,427	$ 39,900
State and Local									934	764	638
Subtotal									28,702	43,191	40,538
Deferred Income Tax Expense (Benefit):
Federal and Foreign									(14,366)	(8,879)	(5,755)
State and local									0	0	0
Subtotal									(14,366)	(8,879)	(5,755)
Actual Tax Expense and Rate	$ 879	$ (462)	$ 7,684	$ 6,235	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 14,336	$ 34,312	$ 34,783