Stockholders' equity Accumulated other comprehensive loss (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2012	$ (24,833)
Other comprehensive income (loss) before reclassifications	(25,610)
Amounts reclassified from other comprehensive income	14,239
Total other comprehensive (loss) income	(11,371)
Balance as of December 31, 2013	(36,204)
Unrealized Gain on Marketable Securities
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2012	1,473
Other comprehensive income (loss) before reclassifications	1,300
Amounts reclassified from other comprehensive income	(1,183)
Total other comprehensive (loss) income	117
Balance as of December 31, 2013	1,590
Unrealized Loss on Interest Rate Swap Agreements
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2012	(20,661)
Other comprehensive income (loss) before reclassifications	1,918
Amounts reclassified from other comprehensive income	15,422
Total other comprehensive (loss) income	17,340
Balance as of December 31, 2013	(3,321)
Unrealized Loss on Foreign Currency Translation
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2012	(5,645)
Other comprehensive income (loss) before reclassifications	(28,828)
Amounts reclassified from other comprehensive income	0
Total other comprehensive (loss) income	(28,828)
Balance as of December 31, 2013	$ (34,473)