Consolidated Statement of Changes in Retained Earnings and Accumulated Other Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 561,212
Net income	26,956	$ 93,862	$ 67,944
Pension and post-retirement plan adjustments, net of tax	20,120	1,321	6,181
Loss (income) attributable to noncontrolling interests	(3,694)	(3,578)	4,280
Balance at end of period	602,578	561,212
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	901,415	858,553	835,126
Net income	26,956	93,862	67,944
Foreign currency translation adjustments	0	0	0
Pension and post-retirement plan adjustments, net of tax	0	0	0
Reclassification from accumulated other comprehensive (loss) into net income	0	0
Loss (income) attributable to noncontrolling interests	(929)	(1,926)	2,863
Common dividends	(52,495)	(49,032)	(47,338)
Preferred dividends	(42)	(42)	(42)
Balance at end of period	868,675	901,415	858,553
Accumulated Other Comprehensive (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(230,246)	(208,199)	(166,730)
Net income	0	0	0
Foreign currency translation adjustments	41,129	(24,986)	(49,067)
Pension and post-retirement plan adjustments, net of tax	20,120	1,321	6,181
Reclassification from accumulated other comprehensive (loss) into net income	0	3,270
Loss (income) attributable to noncontrolling interests	(2,765)	(1,652)	1,417
Common dividends	0	0	0
Preferred dividends	0	0	0
Cumulative effect of the adoption of ASU 2016-16 (Note 1)	0
Balance at end of period	$ (171,762)	$ (230,246)	$ (208,199)