Fair value measurement - Narrative (Details)	3 Months Ended
	Mar. 31, 2018
Secured credit facilities | Secured debt: | LIBOR
Debt Instrument [Line Items]
Variable rate floor	1.00%
—Euro Term Loan | Secured debt: | LIBOR
Debt Instrument [Line Items]
Variable rate floor	3.00%
—Euro Term Loan | Term loan | EURIBOR
Debt Instrument [Line Items]
Variable rate floor	0.00%