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Employee benefit plans - Components of Net Periodic Pension Expense (Income) and Other Amounts Recognized in Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):      
Net loss (gain) $ 7,863 $ 7,567  
Pension Benefits [Member]      
Components of net periodic pension cost:      
Interest cost 696 678 $ 646
Expected return on plan assets (931) (910) (825)
Amortization of unrecognized net gain loss 198 92 180
Net periodic benefit cost (37) (140) 1
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):      
Net loss (gain) 296 3,684 (2,369)
Deferred tax (104) (1,289) 805
Total recognized in other comprehensive income 192 2,395 (1,564)
Total recognized in net period pension cost and other comprehensive income $ 155 $ 2,255 (1,563)
Postretirement Life Insurance Benefits [Member]      
Components of net periodic pension cost:      
Service cost     35
Interest cost     111
Amortization of unrecognized net gain loss     96
Net periodic benefit cost     242
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):      
Net loss (gain)     (1,273)
Deferred tax     620
Total recognized in other comprehensive income     (653)
Total recognized in net period pension cost and other comprehensive income     $ (411)