Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,520,548	$ 3,249,784
Amounts reclassified from accumulated other comprehensive loss	$ (375)	$ (712)	1,095	(2,024)
Net current-period other comprehensive income (loss)	376	678	(1,058)	1,857
Balance	2,237,610	2,686,892	2,237,610	2,686,892
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,575)	(2,972)	(2,667)	(3,071)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	91	100	183	199
Net current-period other comprehensive income (loss)	91	100	183	199
Balance	(2,484)	(2,872)	(2,484)	(2,872)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(35,852)	(37,843)	(34,257)	(38,218)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	248	584	(1,347)	959
Net current-period other comprehensive income (loss)	248	584	(1,347)	959
Balance	(35,604)	(37,259)	(35,604)	(37,259)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(175)	(139)	(200)	(829)
Other comprehensive income (loss) before reclassifications	31	9	56	(103)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	802
Net current-period other comprehensive income (loss)	31	9	56	699
Balance	(144)	(130)	(144)	(130)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	20	30	(24)	15
Other comprehensive income (loss) before reclassifications	(30)	(43)	(19)	(64)
Amounts reclassified from accumulated other comprehensive loss	36	28	69	64
Net current-period other comprehensive income (loss)	6	(15)	50	0
Balance	26	15	26	15
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(38,582)	(40,924)	(37,148)	(42,103)
Other comprehensive income (loss) before reclassifications	1	(34)	37	(167)
Amounts reclassified from accumulated other comprehensive loss	375	712	(1,095)	2,024
Net current-period other comprehensive income (loss)	376	678	(1,058)	1,857
Balance	$ (38,206)	$ (40,246)	$ (38,206)	$ (40,246)