Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Impairment charges on minority investments		$ 1,743	$ 1,210
Aggregate carrying investment amount		8,263	9,116
Redeemable noncontrolling interests		8,872	8,872
Finite lives impairment charge	$ 93,520	0	0	$ 93,520
Advertising costs		13,683	12,469	15,245
Retained earnings		$ (677,772)	$ (621,787)
Minimum [Member]
Significant Accounting Policies [Line Items]
Sale payment period		30 days
Ownership of companies, percent		20.00%
Maximum [Member]
Significant Accounting Policies [Line Items]
Sale payment period		60 days
Ownership of companies, percent		50.00%
Americas [Member]
Significant Accounting Policies [Line Items]
Fair value of goodwill	$ 0			0
Goodwill impairment				$ 382,271
EMEA [Member]
Significant Accounting Policies [Line Items]
Carrying value of goodwill exceeds fair value, percent	29.00%			29.00%
Goodwill impairment				$ 61,388