Stockholders' equity Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2014			$ (628)
Other comprehensive income (loss) before reclassifications			27,220
Amounts reclassified from other comprehensive (loss) income			8,675
Total other comprehensive (loss) income	$ (48,841)	$ (12,466)	35,895	$ 7,493
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			(29)
Net other comprehensive income (loss)			35,866
Balance as of September 30, 2015	35,238		35,238
Unrealized Gains on Marketable Securities
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2014			52,367
Other comprehensive income (loss) before reclassifications			54,004
Amounts reclassified from other comprehensive (loss) income			(2,503)
Total other comprehensive (loss) income			51,501
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			0
Net other comprehensive income (loss)			51,501
Balance as of September 30, 2015	103,868		103,868
Unrealized Losses on Interest Rate Swap Agreements
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2014			(909)
Other comprehensive income (loss) before reclassifications			(9,712)
Amounts reclassified from other comprehensive (loss) income			1,942
Total other comprehensive (loss) income			(7,770)
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			0
Net other comprehensive income (loss)			(7,770)
Balance as of September 30, 2015	(8,679)		(8,679)
Unrealized Losses on Foreign Currency Translation
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2014			(52,086)
Other comprehensive income (loss) before reclassifications			(17,072)
Amounts reclassified from other comprehensive (loss) income			9,236
Total other comprehensive (loss) income			(7,836)
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			(29)
Net other comprehensive income (loss)			(7,865)
Balance as of September 30, 2015	$ (59,951)		$ (59,951)