Estimated future benefit payments and subsidies (Details 11) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	$ 5,596
Prescription drug subsidy receipts, next twelve months	237
Expected future benefit payments, year two	5,584
Prescription drug subsidy receipts, year two	230
Expected future benefit payments, year three	5,583
Prescription drug subsidy receipts, year three	221
Expected future benefit payments, year four	5,543
Prescription drug subsidy receipts, Year Four	211
Expected future benefit payments, year five	5,483
Prescription drug subsidy receipts, year five	200
Expected future benefit payments, five fiscal years thereafter	26,038
Prescription drug subsidy receipts, after year five	823
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	23,391
Expected future benefit payments, year two	23,645
Expected future benefit payments, year three	23,911
Expected future benefit payments, year four	24,439
Expected future benefit payments, year five	24,814
Expected future benefit payments, five fiscal years thereafter	$ 130,026