Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated other comprehensive loss [Roll Forward]
Balance	$ 3,198,802	$ 3,100,306	$ 3,079,105	$ 2,897,687	$ 2,836,592	$ 2,847,246	$ 3,079,105	$ 2,847,246
Other comprehensive income (loss)	543	542	568	560	586	707	1,653	1,853
Balance	3,332,278	3,198,802	3,100,306	3,012,765	2,897,687	2,836,592	3,332,278	3,012,765
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive loss [Roll Forward]
Balance	(761)	(873)	(984)	(1,207)	(1,319)	(1,430)	(984)	(1,430)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	111	112	111	111	112	111
Other comprehensive income (loss)	111	112	111	111	112	111
Balance	(650)	(761)	(873)	(1,096)	(1,207)	(1,319)	(650)	(1,096)
Postretirement liability adjustment
Accumulated other comprehensive loss [Roll Forward]
Balance	(46,284)	(46,741)	(47,207)	(39,792)	(40,272)	(40,734)	(47,207)	(40,734)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Amounts reclassified from (to) accumulated other comprehensive loss	466	457	466	471	480	462
Other comprehensive income (loss)	466	457	466	471	480	462
Balance	(45,818)	(46,284)	(46,741)	(39,321)	(39,792)	(40,272)	(45,818)	(39,321)
Net unrealized gain (loss) on available-for-sale investments
Accumulated other comprehensive loss [Roll Forward]
Balance	77	104	113	190	196	62	113	62
Other comprehensive income (loss) before reclassifications	(54)	(52)	(44)	(50)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	20	25	35	28	7	(1)
Other comprehensive income (loss)	(34)	(27)	(9)	(22)	(6)	134
Balance	43	77	104	168	190	196	43	168
Total accumulated other comprehensive loss
Accumulated other comprehensive loss [Roll Forward]
Balance	(46,968)	(47,510)	(48,078)	(40,809)	(41,395)	(42,102)	(48,078)	(42,102)
Other comprehensive income (loss) before reclassifications	(54)	(52)	(44)	(50)	(13)	135
Amounts reclassified from (to) accumulated other comprehensive loss	597	594	612	610	599	572
Other comprehensive income (loss)	543	542	568	560	586	707
Balance	$ (46,425)	$ (46,968)	$ (47,510)	$ (40,249)	$ (40,809)	$ (41,395)	$ (46,425)	$ (40,249)

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669619-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118261656&loc=d3e32136-110900

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=121640130&loc=d3e1436-108581

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=118261656&loc=d3e32211-110900

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450520&loc=d3e32583-110901

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669625-108580

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

+ References + Details