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Employee benefit plans (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Pension benefits [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost $ 32 $ 39 $ 96 $ 116
Interest cost 4,420 4,062 13,265 12,186
Expected return on assets (5,304) (4,979) (15,913) (14,937)
Amortization of prior service cost (credit) 18 18 54 54
Amortization of net actuarial loss 1,217 1,793 3,651 5,373
Net periodic benefit cost (credit), including amount capitalized 383 933 1,153 2,792
Less amount capitalized 27 157 195 425
Net periodic benefit cost 356 776 958 2,367
Other postretirement benefits [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost 379 419 1,138 1,257
Interest cost 919 804 2,701 2,411
Expected return on assets (1,154) (1,086) (3,463) (3,258)
Amortization of prior service cost (credit) (348) (364) (1,044) (1,092)
Amortization of net actuarial loss 162 327 486 1,405
Net periodic benefit cost (credit), including amount capitalized (42) 100 (182) 723
Less amount capitalized (65) 47 (55) 137
Net periodic benefit cost 23 53 (127) 586
Supplemental employee retirement plans [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Net periodic benefit cost $ 1,700 $ 1,800 $ 5,000 $ 5,500