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Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ 3,100,306	$ 3,079,105	$ 2,836,592	$ 2,847,246	$ 3,079,105	$ 2,847,246
Other comprehensive income (loss)	542	568	586	707	1,110	1,293
Balance	3,198,802	3,100,306	2,897,687	2,836,592	3,198,802	2,897,687
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(873)	(984)	(1,319)	(1,430)	(984)	(1,430)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	112	111	112	111
Other comprehensive income (loss)	112	111	112	111
Balance	(761)	(873)	(1,207)	(1,319)	(761)	(1,207)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(46,741)	(47,207)	(40,272)	(40,734)	(47,207)	(40,734)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	457	466	480	462
Other comprehensive income (loss)	457	466	480	462
Balance	(46,284)	(46,741)	(39,792)	(40,272)	(46,284)	(39,792)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	104	113	196	62	113	62
Other comprehensive income (loss) before reclassifications	(52)	(44)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	25	35	7	(1)
Other comprehensive income (loss)	(27)	(9)	(6)	134
Balance	77	104	190	196	77	190
Total accumulated other comprehensive loss
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(47,510)	(48,078)	(41,395)	(42,102)	(48,078)	(42,102)
Other comprehensive income (loss) before reclassifications	(52)	(44)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	594	612	599	572
Other comprehensive income (loss)	542	568	586	707
Balance	$ (46,968)	$ (47,510)	$ (40,809)	$ (41,395)	$ (46,968)	$ (40,809)

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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