Segments Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation From Segment Totals To Consolidated Abstract
Operating income for reportable segments	$ 165,313				$ 158,895				$ 165,313	$ 158,895	$ 155,367
Non-operating Charges									65,142	65,316	66,999
Restructuring and related activities									0	439	(6,790)
Combination-related expenses									(29,938)	(1,531)	0
Depreciation of corporate assets and amortization									(7,489)	(7,134)	(7,010)
Operating income	$ 17,074	$ 14,009	$ 17,903	$ 13,758	$ 20,951	$ 21,863	$ 22,701	$ 19,838	62,744	85,353	74,568
Interest expense									(3,892)	(2,889)	(2,585)
Interest income									2,534	2,037	1,624
Other expense, net									(718)	(492)	(3,377)
Income before taxes and equity in net income of associated companies									$ 60,668	$ 84,009	$ 70,230