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Employee benefit plans (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Pension plans, defined benefit [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Defined benefit plan, service cost $ 349 $ 35 $ 1,044 $ 1,689
Defined benefit plan, interest cost 4,407 4,706 13,223 14,625
Defined benefit plan, expected return on plan assets (5,865) (5,679) (17,596) (17,106)
Defined benefit plan, amortization of prior service cost (credit) (22) (54) (64) 33
Defined benefit plan, amortization of net actuarial (gain) loss 1,887 917 5,670 3,509
Defined benefit plan, amortization of net transition obligation 0 0 0 0
Curtailment (gain) loss (1,023) 0 (1,023) 1,218
Net periodic benefit cost, including amount capitalized (267) (75) 1,254 3,968
Net periodic benefit cost amount capitalized 185 323 615 858
Net periodic benefit cost (452) (398) 639 3,110
Other postretirement benefit plans, defined benefit [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Defined benefit plan, service cost 437 361 1,310 1,083
Defined benefit plan, interest cost 943 1,175 3,124 3,525
Defined benefit plan, expected return on plan assets (1,222) (1,263) (3,667) (3,789)
Defined benefit plan, amortization of prior service cost (credit) (534) (669) (1,078) (2,007)
Defined benefit plan, amortization of net actuarial (gain) loss 356 430 1,769 688
Defined benefit plan, amortization of net transition obligation 531 532 1,594 1,594
Curtailment (gain) loss 0 0 0 0
Net periodic benefit cost, including amount capitalized 511 566 3,052 1,094
Net periodic benefit cost amount capitalized 314 (41) 635 (136)
Net periodic benefit cost 197 607 2,417 1,230
Supplemental employee retirement plans, defined benefit [Member]
        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Net periodic benefit cost $ 2,000 $ 2,000 $ 6,100 $ 6,000