Significant accounting policies (new accounting pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Standards Update No. 2014-09 [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
Capitalized contract costs amortization period, practical expedient		1 year
Accounting Standards Update No. 2014-09 [Member] | Minimum [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
Capitalized contract costs amortization period		3 years
Accounting Standards Update No. 2014-09 [Member] | Maximum [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
Capitalized contract costs amortization period		6 years
Accounting Standards Update No. 2014-09 [Member] | Other non-current assets [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption	$ 6,000
Accounting Standards Update No. 2014-09 [Member] | Retained earnings [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption	6,000
Accounting Standards Update No. 2017-07 [Member] | Operating income [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption			$ (2,016)	$ (1,841)	$ (2,697)
Accounting Standards Update No. 2017-07 [Member] | Other income [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption			$ 2,016	$ 1,841	$ 2,697
Accounting Standards Update No. 2018-02 [Member] | Retained earnings [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption	7,000
Accounting Standards Update No. 2018-02 [Member] | Accumulated other comprehensive loss [Member] | Forecast [Member]
New accounting pronouncements [Line Items]
New accounting pronouncement, effect of adoption	$ (7,000)