Estimated future benefit payments and subsidies (Details 12) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	$ 6,099
Prescription drug subsidy receipts, next twelve months	256
Expected future benefit payments, year two	6,134
Prescription drug subsidy receipts, year two	248
Expected future benefit payments, year three	6,127
Prescription drug subsidy receipts, year three	239
Expected future benefit payments, year four	6,082
Prescription drug subsidy receipts, Year Four	228
Expected future benefit payments, year five	6,083
Prescription drug subsidy receipts, year five	217
Expected future benefit payments, five fiscal years thereafter	29,051
Prescription drug subsidy receipts, after year five	896
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next twelve months	23,193
Expected future benefit payments, year two	23,386
Expected future benefit payments, year three	23,646
Expected future benefit payments, year four	23,954
Expected future benefit payments, year five	24,531
Expected future benefit payments, five fiscal years thereafter	$ 128,971