Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	$ 350	$ 827	$ 695	$ 1,654
Defined benefit plan, interest cost	4,262	4,959	8,816	9,919
Defined benefit plan, expected return on plan assets	(5,845)	(5,727)	(11,731)	(11,427)
Defined benefit plan, amortization of prior service cost (credit)	(21)	44	(42)	87
Defined benefit plan, amortization of net actuarial (gain) loss	2,102	1,049	3,783	2,592
Defined benefit plan, amortization of net transition obligation	0	0	0	0
Curtailment loss	0	1,218	0	1,218
Net periodic benefit cost, including amount capitalized	848	2,370	1,521	4,043
Net periodic benefit cost amount capitalized	196	287	430	535
Net periodic benefit cost	652	2,083	1,091	3,508
Other postretirement benefit plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	461	383	873	722
Defined benefit plan, interest cost	1,038	1,161	2,181	2,350
Defined benefit plan, expected return on plan assets	(1,201)	(1,308)	(2,445)	(2,526)
Defined benefit plan, amortization of prior service cost (credit)	(272)	(669)	(544)	(1,338)
Defined benefit plan, amortization of net actuarial (gain) loss	887	(53)	1,413	258
Defined benefit plan, amortization of net transition obligation	531	531	1,063	1,062
Curtailment loss	0	0	0	0
Net periodic benefit cost, including amount capitalized	1,444	45	2,541	528
Net periodic benefit cost amount capitalized	183	(28)	321	(95)
Net periodic benefit cost	1,261	73	2,220	623
Supplemental employee retirement plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 2,000	$ 1,900	$ 4,100	$ 4,000