Comprehensive income (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive income and components of other comprehensive income (loss) [Abstract]
Net income	$ 63,974	$ 61,010	$ 152,019	$ 151,719
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	32,547	3,628	31,787	17,266
Less: Reclassification adjustment for gain (loss) on derivative instruments included in net income, net of tax	(534)	5,348	77	(2,797)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	33,081	(1,720)	31,710	20,063
Foreign currency translation adjustment, net of tax	(1,401)	2,679	(1,140)	1,266
Net unrealized gains on available-for-sale investments, net of tax			103	0
Other comprehensive income (loss), net of tax	31,680	959	30,673	21,329
Comprehensive income	95,654	61,969	182,692	173,048
Other comprehensive income (loss), tax [Abstract]
Net unrealized gain (loss) on derivative instruments arising during the period, tax	19,481	2,177	19,367	10,351
Reclassification adjustment for gain (loss) on derivative instruments included in net income, tax	(320)	3,209	45	(1,745)
Foreign currency translation adjustment, tax	(905)	1,730	(736)	801
Net unrealized gains on available-for-sale investments, tax			$ 56	$ 0