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Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Interest expense $ (20,303) $ (22,868)
Income taxes (12,188) (8,301)
Other income 1,017 1,049
Net income 37,325 13,829
Reclassification from accumulated other comprehensive income, current period, net of tax (483) 1,467
Reclassification adjustment for loss on derivative instruments included in net income    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Reclassification from accumulated other comprehensive income, current period, net of tax 91 92
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive income [Member] | Interest rate contract [Member]    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Interest expense (147) (149)
Income taxes 56 57
Net income (91) (92)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Reclassification from accumulated other comprehensive income, current period, net of tax 357 (1,595)
Amortization of postretirement liability gains (losses) included in net periodic benefit cost | Reclassification out of accumulated other comprehensive income [Member]    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Income taxes 217 (969)
Net periodic benefit cost (574) 2,564
Net income (357) 1,595
Reclassification adjustment for loss on available-for-sale investments included in net income    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Reclassification from accumulated other comprehensive income, current period, net of tax 35 36
Reclassification adjustment for loss on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive income [Member]    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Income taxes 19 19
Other income (54) (55)
Net income $ (35) $ (36)