Schedule I - Condensed Statements of Comprehensive Income (Parent Company Only) (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax expense (benefit) on actuarial gain (loss), employee benefit plans	$ (3,507)	$ (5,205)	$ 5,644
Tax expense, adjustment for items reclassified to earnings, employee benefit plans	2,220	1,757	1,507
Parent Company [Member]
Tax expense (benefit) on actuarial gain (loss), employee benefit plans	(3,507)	(5,205)	5,644
Tax expense, adjustment for items reclassified to earnings, employee benefit plans	$ 2,220	$ 1,757	$ 1,507