Accumulated Other Comprehensive Loss - Summary of Income Tax Expense (Benefit) Associated with Various Components of Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income tax expense (benefit) associated with changes in:
Unrecognized employee benefit costs	$ 10,265	$ (15,890)	$ 5,256
Unrealized holding gains on marketable securities	(47)	(232)	(80)
Fair value of cash flow hedges	206	149	(61)
Income tax expense (benefit) associated with reclassification adjustments:
Amortization of unrecognized employee benefit costs	(1,841)	(3,156)	(3,547)
Realized gains from sale of marketable securities	60	30	42
Realized gains/losses from settlement of cash flow hedges	$ (57)	$ (146)	$ (67)