Components of net periodic benefit cost (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,078	$ 2,252	$ 2,889
Interest cost	17,598	19,500	19,761
Expected return on assets	(23,536)	(22,809)	(23,643)
Amortization of prior service cost (credit)	(46)	45	152
Recognized net actuarial loss	7,070	4,656	2,622
Curtailment loss (gain)	(1,023)	1,218	0
Amortization of net transition obligation	0	0	0
Net periodic benefit cost, including amount capitalized	1,141	4,862	1,781
Less amount capitalized	937	1,196	791
Net periodic benefit cost	204	3,666	990
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net loss	19,982	76,310	20,477
Prior service cost (credit)	0	0	353
Amortization of actuarial loss	(7,070)	(4,656)	(2,622)
Amortization of prior service (cost) credit	1,069	(1,263)	(152)
Amortization of net transition obligation	0	0	0
Total recognized in accumulated other comprehensive (income) loss	13,981	70,391	18,056
Total recognized in net periodic benefit cost and accumulated other comprehensive (income) loss	14,185	74,057	19,046
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,747	1,443	1,357
Interest cost	4,166	4,700	4,817
Expected return on assets	(4,890)	(5,051)	(5,512)
Amortization of prior service cost (credit)	(1,438)	(2,677)	(3,303)
Recognized net actuarial loss	2,134	753	845
Curtailment loss (gain)	0	0	0
Amortization of net transition obligation	2,128	2,125	2,125
Net periodic benefit cost, including amount capitalized	3,847	1,293	329
Less amount capitalized	910	(50)	(92)
Net periodic benefit cost	2,937	1,343	421
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net loss	1,863	23,863	1,462
Prior service cost (credit)	(11,418)	0	121
Amortization of actuarial loss	(2,134)	(753)	(845)
Amortization of prior service (cost) credit	1,438	2,677	3,303
Amortization of net transition obligation	(2,128)	(2,125)	(2,125)
Total recognized in accumulated other comprehensive (income) loss	(12,379)	23,662	1,916
Total recognized in net periodic benefit cost and accumulated other comprehensive (income) loss	$ (9,442)	$ 25,005	$ 2,337