Stockholders' equity Accumulated other comprehensive loss (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2013	$ (36,204)
Other comprehensive income (loss) before reclassifications	21,265
Amounts reclassified from other comprehensive income	(3,490)
Net other comprehensive income (loss)	17,775
Balance as of March 31, 2014	(18,429)
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 (loss) before reclassifications	18,779
Amounts reclassified from other comprehensive income	0
Net other comprehensive income (loss)	18,779
Balance as of March 31, 2014	20,369
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 (loss) before reclassifications	5,592
Amounts reclassified from other comprehensive income	(3,490)
Net other comprehensive income (loss)	2,102
Balance as of March 31, 2014	(1,219)
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 (loss) before reclassifications	(3,106)
Amounts reclassified from other comprehensive income	0
Net other comprehensive income (loss)	(3,106)
Balance as of March 31, 2014	$ (37,579)