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Employee benefit plans (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Pension benefits [Member]    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Service cost $ 0 $ 40
Interest cost 4,390 4,364
Expected return on assets (5,280) (5,373)
Amortization of prior service cost (credit) 0 18
Amortization of net actuarial loss 1,593 1,735
Net periodic benefit cost, including amount capitalized 703 784
Less amount capitalized 81 76
Net periodic benefit cost 622 708
Other postretirement benefits [Member]    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Service cost 450 483
Interest cost 949 914
Expected return on assets (1,149) (1,175)
Amortization of prior service cost (credit) (343) (342)
Amortization of net actuarial loss 448 461
Net periodic benefit cost, including amount capitalized 355 341
Less amount capitalized 34 29
Net periodic benefit cost 321 312
Supplemental employee retirement plans [Member]    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Net periodic benefit cost 1,900 $ 1,700
Curtailment gain $ 3,300