Components of Current and Deferred Income Tax Expense (Benefit) (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
Current Income Tax Expense (Benefit):
Federal and Foreign									$ 42,427	$ 39,900	$ 27,937
State and Local									764	638	1,476
Subtotal									43,191	40,538	29,413
Deferred Income Tax Expense (Benefit):
Federal and Foreign									(8,879)	(5,755)	7,004
State and local									0	0	0
Subtotal									(8,879)	(5,755)	7,004
Actual Tax Expense and Rate	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 9,866	$ 7,875	$ 7,053	$ 9,989	$ 34,312	$ 34,783	$ 36,417