Reclassification out of accumulated other comprehensive income (loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (20,447)	$ (20,303)
Income taxes	(7,551)	(12,188)
Other income	582	2,343
Net income	42,437	37,325
Reclassification from accumulated other comprehensive income, current period, net of tax	(540)	(483)
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	92	91
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive income | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	(148)	(147)
Income taxes	56	56
Net income	(92)	(91)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	418	357
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	155	217
Other income	(573)	(574)
Net income	(418)	(357)
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	30	35
Reclassification adjustment for loss on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income taxes	7	19
Other income	(37)	(54)
Net income	$ (30)	$ (35)