Benefit Plans - Component of Net Periodic Pension Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 33,602	$ 30,140
Deferred tax benefit	(14,044)	(12,647)
Total	$ 19,558	$ 17,493