Other comprehensive income (loss) - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Other comprehensive income (loss)
Net unrealized gain on investment securities available-for-sale	$ 826	$ 553
Income tax	289	193
Net of income taxes	537	360
Benefit plan adjustments	(6,946)	(6,946)
Income tax expense (benefit)	(2,431)	(2,431)
Net of income taxes	(4,515)	(4,515)
Accumulated other comprehensive loss	$ (3,978)	$ (4,155)