Accumulated Other Comprehensive Loss - Summary of Income Tax Expense (Benefit) Associated with Various Components of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax expense (benefit) associated with changes in:
Unrecognized employee benefit costs	$ 0	$ 0	$ (4,731)
Unrealized holding losses on marketable securities	0	0	134
Fair value of cash flow hedges	0	0	294
Income tax expense (benefit) associated with reclassification adjustments:
Amortization of unrecognized employee benefit costs	0	0	(2,573)
Realized gains from sale of marketable securities	0	366	29
Realized losses from settlement of cash flow hedges	$ 0	$ (79)	$ (272)