Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	$ 40	$ 345
Defined benefit plan, interest cost	4,018	4,554
Defined benefit plan, expected return on plan assets	(5,083)	(5,886)
Defined benefit plan, amortization of prior service cost (credit)	18	(21)
Defined benefit plan, amortization of net actuarial (gain) loss	1,864	1,681
Defined benefit plan, amortization of net transition obligation	0	0
Net periodic benefit cost, including amount capitalized	857	673
Net periodic benefit cost amount capitalized	110	234
Net periodic benefit cost	747	439
Other postretirement benefit plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	504	412
Defined benefit plan, interest cost	940	1,143
Defined benefit plan, expected return on plan assets	(1,107)	(1,244)
Defined benefit plan, amortization of prior service cost (credit)	(364)	(272)
Defined benefit plan, amortization of net actuarial (gain) loss	671	526
Defined benefit plan, amortization of net transition obligation	0	532
Net periodic benefit cost, including amount capitalized	644	1,097
Net periodic benefit cost amount capitalized	29	138
Net periodic benefit cost	615	959
Supplemental employee retirement plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,900	$ 2,100