Shareholders' equity - change in accumulated other comprehensive income (loss), net of tax (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss), net of tax	$ (438,717)
Ending balance, accumulated other comprehensive income (loss), net of tax	(265,091)	$ (438,717)
Acquisition of noncontrolling interest		(9,231)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss), net of tax	(2,424)	(2,491)
Other comprehensive income (loss) before reclassifications	2,775	(3,434)
Amounts reclassified from accumulated other comprehensive income (loss)	(11)	3,501
Net current-year other comprehensive income (loss)	2,764	67
Ending balance, accumulated other comprehensive income (loss), net of tax	340	(2,424)
Pension and Other Postretirement Benefit Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss), net of tax	(136,596)	(138,887)
Other comprehensive income (loss) before reclassifications	(6,725)	(2,221)
Amounts reclassified from accumulated other comprehensive income (loss)	4,513	4,512
Net current-year other comprehensive income (loss)	(2,212)	2,291
Ending balance, accumulated other comprehensive income (loss), net of tax	(138,808)	(136,596)
Foreign Currency Translation Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss), net of tax	(299,697)	(229,746)
Other comprehensive income (loss) before reclassifications	173,074	(69,119)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Net current-year other comprehensive income (loss)	173,074	(69,119)
Ending balance, accumulated other comprehensive income (loss), net of tax	(126,623)	(299,697)
Acquisition of noncontrolling interest		(832)
Accumulated Other Comprehensive Income (loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss), net of tax	(438,717)	(371,124)
Other comprehensive income (loss) before reclassifications	169,124	(74,774)
Amounts reclassified from accumulated other comprehensive income (loss)	4,502	8,013
Net current-year other comprehensive income (loss)	173,626	(66,761)
Ending balance, accumulated other comprehensive income (loss), net of tax	$ (265,091)	(438,717)
Acquisition of noncontrolling interest		$ (832)