Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	$ 37	$ 350	$ 77	$ 695
Defined benefit plan, interest cost	4,106	4,262	8,124	8,816
Defined benefit plan, expected return on plan assets	(4,875)	(5,845)	(9,958)	(11,731)
Defined benefit plan, amortization of prior service cost (credit)	18	(21)	36	(42)
Defined benefit plan, amortization of net actuarial (gain) loss	1,716	2,102	3,580	3,783
Defined benefit plan, amortization of net transition obligation	0	0	0	0
Net periodic benefit cost, including amount capitalized	1,002	848	1,859	1,521
Net periodic benefit cost amount capitalized	158	196	268	430
Net periodic benefit cost	844	652	1,591	1,091
Other postretirement benefit plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Defined benefit plan, service cost	334	461	838	873
Defined benefit plan, interest cost	667	1,038	1,607	2,181
Defined benefit plan, expected return on plan assets	(1,065)	(1,201)	(2,172)	(2,445)
Defined benefit plan, amortization of prior service cost (credit)	(364)	(272)	(728)	(544)
Defined benefit plan, amortization of net actuarial (gain) loss	407	887	1,078	1,413
Defined benefit plan, amortization of net transition obligation	0	531	0	1,063
Net periodic benefit cost, including amount capitalized	(21)	1,444	623	2,541
Net periodic benefit cost amount capitalized	61	183	90	321
Net periodic benefit cost	(82)	1,261	533	2,220
Supplemental employee retirement plans, defined benefit [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,800	$ 2,000	$ 3,600	$ 4,100