Defined Contribution Plans, Pension and Other Post-Retirement Benefit Plans - Fair Value of Pension Plan Assets Measured Using Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Fair value of plan assets — Beginning of the year	$ 69,470
Fair value of plan assets at end of the year	80,423	$ 69,470
Level 3
Change in plan assets:
Fair value of plan assets — Beginning of the year	9,409	8,645
Actual return on assets held at reporting date	744	764
Fair value of plan assets at end of the year	$ 10,153	$ 9,409