Comprehensive income (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Comprehensive income and components of other comprehensive income (loss) [Abstract]
Net income	$ 45,067	$ 48,938	$ 88,044	$ 90,710
Net unrealized gain (loss) on derivative instruments qualifying as hedges [Abstract]
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax	17,057	4,637	(1,217)	19,932
Less: Reclassification adjustment for gain (loss) on derivative instruments included in net income, net of tax	(3,650)	5,259	155	(1,850)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	20,707	(622)	(1,372)	21,782
Foreign currency translation adjustment, net of tax	50	(476)	262	(1,412)
Net unrealized gains on available-for-sale investments, net of tax	87	0	103	0
Other comprehensive income (loss), net of tax	20,844	(1,098)	(1,007)	20,370
Comprehensive income	65,911	47,840	87,037	111,080
Other comprehensive income (loss), tax [Abstract]
Net unrealized gain (loss) on derivative instruments arising during the period, tax	10,576	2,588	(388)	11,962
Reclassification adjustment for gain (loss) on derivative instruments included in net income, tax	(2,191)	3,191	91	(1,166)
Foreign currency translation adjustment, tax	32	(307)	170	(929)
Net unrealized gains on available-for-sale investments, tax	$ 47		$ 55