Changes in Accumulated OCI (Loss) by Component (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Changes in AOCI by Component
Beginning balance		$ 9,633,878	$ 11,139,588	$ 9,803,588	$ 10,958,229
Change in accounting principle		72	118	84	(260)
Ending balance		9,438,827	11,248,475	9,438,827	11,248,475
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		(5,187)	(5,784)	(5,199)	(5,406)
Losses reclassified from accumulated OCI, net of tax		72	118	84	236
Ending balance		(5,115)	(5,666)	(5,115)	(5,666)
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	(345)	0	(387)
Losses reclassified from accumulated OCI, net of tax		0	42	0	84
Ending balance		0	(303)	0	(303)
Other postretirement benefits liability adjustment, net of tax
Changes in AOCI by Component
Beginning balance		(5,187)	(5,439)	(5,199)	(5,019)
Losses reclassified from accumulated OCI, net of tax		72	76	84	152
Ending balance		$ (5,115)	$ (5,363)	$ (5,115)	(5,363)
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.