Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			$ 335,322
Other comprehensive income (loss)	$ (5,199)	$ 317	(1,698)	$ 1,297
Balance at end of period	305,532		305,532
Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		18,751	46,069	20,473
Change in accounting principle - ASU 2016-01			(46,069)
Balance at beginning of period, as adjusted		18,751		20,473
Other comprehensive income (loss) - unrealized gains (losses) arising during the year		(2,869)		(4,591)
Balance at end of period		15,882		15,882
Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(161,876)	(174,088)	(164,467)	(175,859)
Other comprehensive income (loss)	(6,108)	2,925	(3,517)	4,696
Balance at end of period	(167,984)	(171,163)	(167,984)	(171,163)
Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(278)		(390)
Unrealized gains (losses) arisingduring the year		(178)		(177)
Less reclassification adjustment foramounts included in interest expense		94		205
Balance at end of period		(362)		(362)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(71,968)	(75,843)	(72,951)	(76,710)
Other comprehensive income (loss)	974	393	1,957	1,260
Balance at end of period	(70,994)	(75,450)	(70,994)	(75,450)
Accumulated Defined Benefit Plans Adjustment | OPEB
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(461)	(408)	(388)	(360)
Other comprehensive income (loss)	(65)	(48)	(138)	(96)
Balance at end of period	(526)	(456)	(526)	(456)
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(234,305)	(231,866)	(191,737)	(232,846)
Change in accounting principle - ASU 2016-01			(46,069)
Balance at beginning of period, as adjusted	(234,305)	(231,866)	(237,806)	(232,846)
Other comprehensive income (loss)	(5,199)	317	(1,698)	1,297
Balance at end of period	$ (239,504)	$ (231,549)	$ (239,504)	$ (231,549)