Fair value measurements (recurring fair value measurements) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Recurring fair value measurements [Line Items]
Available-for-sale marketable securities (cash equivalents)	$ 65,985	$ 70,001
Available-for-sale marketable securities (funds held for customers)	9,769	14,736
Available-for-sale marketable securities (other current assets)	2,046	2,045
Long-term investment in mutual funds	2,269	2,407
Derivative assets	1,234	2,158
Derivative liabilities	(10,377)	(16,239)
Transfers between fair value levels	0
Quoted prices in active markets for identical assets (Level 1) [Member]
Recurring fair value measurements [Line Items]
Available-for-sale marketable securities (cash equivalents)	65,985	70,001
Available-for-sale marketable securities (funds held for customers)	0	0
Available-for-sale marketable securities (other current assets)	0	0
Long-term investment in mutual funds	2,269	2,407
Derivative assets	0	0
Derivative liabilities	0	0
Significant other observable inputs (Level 2) [Member]
Recurring fair value measurements [Line Items]
Available-for-sale marketable securities (cash equivalents)	0	0
Available-for-sale marketable securities (funds held for customers)	9,769	14,736
Available-for-sale marketable securities (other current assets)	2,046	2,045
Long-term investment in mutual funds	0	0
Derivative assets	1,234	2,158
Derivative liabilities	(10,377)	(16,239)
Significant unobservable inputs (Level 3) [Member]
Recurring fair value measurements [Line Items]
Available-for-sale marketable securities (cash equivalents)	0	0
Available-for-sale marketable securities (funds held for customers)	0	0
Available-for-sale marketable securities (other current assets)	0	0
Long-term investment in mutual funds	0	0
Derivative assets	0	0
Derivative liabilities	$ 0	$ 0