Stockholders' equity Accumulated other comprehensive loss (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2013	$ (36,204)
Other comprehensive income before reclassifications	1,433
Amounts reclassified from other comprehensive income	6,060
Net other comprehensive income	7,493
Balance as of June 30, 2014	(28,711)
Unrealized Gain on Marketable Securities
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2013	1,590
Other comprehensive income before reclassifications	13,591
Amounts reclassified from other comprehensive income	517
Net other comprehensive income	14,108
Balance as of June 30, 2014	15,698
Unrealized Loss on Interest Rate Swap Agreements
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2013	(3,321)
Other comprehensive income before reclassifications	(2,708)
Amounts reclassified from other comprehensive income	5,742
Net other comprehensive income	3,034
Balance as of June 30, 2014	(287)
Unrealized Loss on Foreign Currency Translation
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2013	(34,473)
Other comprehensive income before reclassifications	(9,450)
Amounts reclassified from other comprehensive income	(199)
Net other comprehensive income	(9,649)
Balance as of June 30, 2014	$ (44,122)