Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Pension plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	$ 345	$ 827
Defined benefit plan, interest cost	4,554	4,960
Defined benefit plan, expected return on plan assets	(5,886)	(5,700)
Defined benefit plan, amortization of prior service cost (credit)	(21)	43
Defined benefit plan, amortization of net actuarial (gain) loss	1,681	1,543
Defined benefit plan, amortization of net transition obligation	0	0
Defined benefit plan net periodic benefit cost including amount capitalized	673	1,673
Defined benefit plan net periodic benefit cost amount capitalized	234	248
Defined benefit plan, net periodic benefit cost	439	1,425
Other postretirement benefit plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	412	339
Defined benefit plan, interest cost	1,143	1,189
Defined benefit plan, expected return on plan assets	(1,244)	(1,218)
Defined benefit plan, amortization of prior service cost (credit)	(272)	(669)
Defined benefit plan, amortization of net actuarial (gain) loss	526	311
Defined benefit plan, amortization of net transition obligation	532	531
Defined benefit plan net periodic benefit cost including amount capitalized	1,097	483
Defined benefit plan net periodic benefit cost amount capitalized	138	(67)
Defined benefit plan, net periodic benefit cost	959	550
Supplemental employee retirement plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, net periodic benefit cost	$ 2,100	$ 2,100