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Employee benefit plans (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Pension benefits [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost $ 39 $ 349 $ 116 $ 1,044
Interest cost 4,062 4,407 12,186 13,223
Expected return on assets (4,979) (5,865) (14,937) (17,596)
Amortization of prior service cost (credit) 18 (22) 54 (64)
Amortization of net actuarial loss 1,793 1,887 5,373 5,670
Amortization of net transition obligation 0 0 0 0
Curtailment gain 0 (1,023) 0 (1,023)
Net periodic benefit cost, including amount capitalized 933 (267) 2,792 1,254
Less amount capitalized 157 185 425 615
Net periodic benefit cost (credit) 776 (452) 2,367 639
Other postretirement benefits [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost 419 437 1,257 1,310
Interest cost 804 943 2,411 3,124
Expected return on assets (1,086) (1,222) (3,258) (3,667)
Amortization of prior service cost (credit) (364) (534) (1,092) (1,078)
Amortization of net actuarial loss 327 356 1,405 1,769
Amortization of net transition obligation 0 531 0 1,594
Curtailment gain 0 0 0 0
Net periodic benefit cost, including amount capitalized 100 511 723 3,052
Less amount capitalized 47 314 137 635
Net periodic benefit cost (credit) 53 197 586 2,417
Supplemental employee retirement plans [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Net periodic benefit cost (credit) $ 1,800 $ 2,000 $ 5,500 $ 6,100