Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 3,249,784	$ 2,855,902
Amounts reclassified from accumulated other comprehensive loss	$ (510)	$ (374)	(2,540)	(1,320)
Net current-period other comprehensive income (loss)	402	166	2,259	1,173
Balance	2,514,871	3,158,503	2,514,871	3,158,503
Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,872)	(3,408)	(3,071)	(3,765)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	100	82	299	439
Net current-period other comprehensive income (loss)	100	82	299	439
Balance	(2,772)	(3,326)	(2,772)	(3,326)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(37,259)	(33,287)	(38,218)	(33,807)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	382	261	1,341	781
Net current-period other comprehensive income (loss)	382	261	1,341	781
Balance	(36,877)	(33,026)	(36,877)	(33,026)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(130)	(579)	(829)	(667)
Other comprehensive loss before reclassifications	(73)	(146)	(176)	(58)
Amounts reclassified from accumulated other comprehensive loss	0	0	802	0
Net current-period other comprehensive income (loss)	(73)	(146)	626	(58)
Balance	(203)	(725)	(203)	(725)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	15	76	15	34
Other comprehensive loss before reclassifications	(35)	(62)	(105)	(89)
Amounts reclassified from accumulated other comprehensive loss	28	31	98	100
Net current-period other comprehensive income (loss)	(7)	(31)	(7)	11
Balance	8	45	8	45
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(40,246)	(37,198)	(42,103)	(38,205)
Other comprehensive loss before reclassifications	(108)	(208)	(281)	(147)
Amounts reclassified from accumulated other comprehensive loss	510	374	2,540	1,320
Net current-period other comprehensive income (loss)	402	166	2,259	1,173
Balance	$ (39,844)	$ (37,032)	$ (39,844)	$ (37,032)