Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,520,548	$ 3,249,784
Amounts reclassified from accumulated other comprehensive loss	$ (361)	$ (510)	730	(2,540)
Net current-period other comprehensive income (loss)	315	402	(743)	2,259
Balance	2,285,061	2,514,871	2,285,061	2,514,871
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,484)	(2,872)	(2,667)	(3,071)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	100	275	299
Net current-period other comprehensive income (loss)	92	100	275	299
Balance	(2,392)	(2,772)	(2,392)	(2,772)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,604)	(37,259)	(34,257)	(38,218)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	236	382	(1,111)	1,341
Net current-period other comprehensive income (loss)	236	382	(1,111)	1,341
Balance	(35,368)	(36,877)	(35,368)	(36,877)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(144)	(130)	(200)	(829)
Other comprehensive income (loss) before reclassifications	(4)	(73)	52	(176)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	802
Net current-period other comprehensive income (loss)	(4)	(73)	52	626
Balance	(148)	(203)	(148)	(203)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	26	15	(24)	15
Other comprehensive income (loss) before reclassifications	(42)	(35)	(65)	(105)
Amounts reclassified from accumulated other comprehensive loss	33	28	106	98
Net current-period other comprehensive income (loss)	(9)	(7)	41	(7)
Balance	17	8	17	8
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,206)	(40,246)	(37,148)	(42,103)
Other comprehensive income (loss) before reclassifications	(46)	(108)	(13)	(281)
Amounts reclassified from accumulated other comprehensive loss	361	510	(730)	2,540
Net current-period other comprehensive income (loss)	315	402	(743)	2,259
Balance	$ (37,891)	$ (39,844)	$ (37,891)	$ (39,844)