Comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated other comprehensive income (loss) [Roll Forward]
Balance			$ 2,316,244	$ 2,520,548
Amounts reclassified from accumulated other comprehensive loss	$ (435)	$ (375)	(918)	1,095
Net current-period other comprehensive income (loss)	396	376	(56)	(1,058)
Balance	2,302,952	2,237,610	2,302,952	2,237,610
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(2,209)	(2,575)	(2,300)	(2,667)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	92	91	183	183
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	92	91	183	183
Balance	(2,117)	(2,484)	(2,117)	(2,484)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,781)	(35,852)	(33,221)	(34,257)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	312	248	669	(1,347)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	312	248	(248)	(1,347)
Balance	(33,469)	(35,604)	(33,469)	(35,604)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(140)	(175)	(149)	(200)
Other comprehensive income (loss) before reclassifications	(15)	31	(6)	56
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	(15)	31	(6)	56
Balance	(155)	(144)	(155)	(144)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(55)	20	(63)	(24)
Other comprehensive income (loss) before reclassifications	(24)	(30)	(51)	(19)
Amounts reclassified from accumulated other comprehensive loss	31	36	66	69
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			0
Net current-period other comprehensive income (loss)	7	6	15	50
Balance	(48)	26	(48)	26
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(36,185)	(38,582)	(35,733)	(37,148)
Other comprehensive income (loss) before reclassifications	(39)	1	(57)	37
Amounts reclassified from accumulated other comprehensive loss	435	375	918	(1,095)
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset			(917)
Net current-period other comprehensive income (loss)	396	376	(56)	(1,058)
Balance	$ (35,789)	$ (38,206)	$ (35,789)	$ (38,206)