Changes in Accumulated OCI (Loss) by Component (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
		Aug. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Changes in AOCI by Component
Beginning balance			$ 9,438,827	$ 11,248,475	$ 9,803,588	$ 10,958,229
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201815Member			us-gaap:AccountingStandardsUpdate201802Member	us-gaap:AccountingStandardsUpdate201802Member
Change in accounting principle			72	120	$ 156	$ (140)
Ending balance			8,845,809	10,897,625	8,845,809	10,897,625
Change in accounting principle
Changes in AOCI by Component
Change in accounting principle	[1]		0	0	0	(496)
Accumulated Other Comprehensive Loss
Changes in AOCI by Component
Beginning balance			(5,115)	(5,666)	(5,199)	(5,406)
Losses reclassified from accumulated OCI, net of tax			72	120	156	356
Ending balance			(5,043)	(5,546)	(5,043)	(5,546)
Accumulated Other Comprehensive Loss | Change in accounting principle
Changes in AOCI by Component
Change in accounting principle						(496)
Interest rate cash flow hedges, net of tax | Change in accounting principle
Changes in AOCI by Component
Change in accounting principle						0
Interest rate cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax
Changes in AOCI by Component
Beginning balance			0	(303)	0	(387)
Losses reclassified from accumulated OCI, net of tax			0	43	0	127
Ending balance			0	(260)	0	(260)
Other postretirement benefits liability adjustment, net of tax
Changes in AOCI by Component
Beginning balance			(5,115)	(5,363)	(5,199)	(5,019)
Losses reclassified from accumulated OCI, net of tax			72	77	156	229
Ending balance			$ (5,043)	$ (5,286)	$ (5,043)	(5,286)
Other postretirement benefits liability adjustment, net of tax | Change in accounting principle
Changes in AOCI by Component
Change in accounting principle						$ (496)

[1] Related to adoption of Accounting Standards Update (ASU) 2018-02.