Income taxes - Narrative (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2017 USD ($)
Income Tax Disclosure [Abstract]
Decrease in income tax expense	$ 62,000
Transition tax on previously unremitted foreign earnings	287,000
Unrecognized tax benefits	152,705
Amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate	100,163
Accrued interest related to the gross unrecognized tax benefits excluded from the unrecognized tax benefits	17,864
Significant decrease in unrecognized tax benefits that is reasonably possible	$ 110,000