Summary of significant accounting policies - Intangible assets (Details) € in Millions	12 Months Ended
	Dec. 31, 2017 EUR (€)
Disclosure of detailed information about intangible assets [line items]
Residual value	€ 0
Computer software | Minimum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	2 years
Computer software | Maximum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	7 years
Customer relationships | Minimum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	5 years
Customer relationships | Maximum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	15 years
Technology | Minimum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	8 years
Technology | Maximum
Disclosure of detailed information about intangible assets [line items]
Useful lives on a straight-line basis	15 years