Changes in Accumulated OCI (Loss) by Component (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Changes in AOCI by Component
Beginning balance		$ 9,803,588	$ 10,958,229
Ending balance		9,633,878	11,139,588
Cumulative Effect, Period of Adoption Balance
Changes in AOCI by Component
Beginning balance	[1]	0
Accumulated Other Comprehensive (Loss) Income
Changes in AOCI by Component
Beginning balance		(5,199)	(5,406)
Losses reclassified from accumulated OCI, net of tax		12	118
Ending balance		(5,187)	(5,784)
Accumulated Other Comprehensive (Loss) Income | Cumulative Effect, Period of Adoption Balance
Changes in AOCI by Component
Beginning balance	[1]	(496)
Interest rate cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax
Changes in AOCI by Component
Beginning balance		0	(387)
Losses reclassified from accumulated OCI, net of tax		0	42
Ending balance		0	(345)
Interest rate cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax | Cumulative Effect, Period of Adoption Balance
Changes in AOCI by Component
Beginning balance		0
Other postretirement benefits liability adjustment, net of tax
Changes in AOCI by Component
Beginning balance		(5,199)	(5,019)
Losses reclassified from accumulated OCI, net of tax		12	76
Ending balance		(5,187)	$ (5,439)
Other postretirement benefits liability adjustment, net of tax | Cumulative Effect, Period of Adoption Balance
Changes in AOCI by Component
Beginning balance		$ (496)

[1] (a)Related to adoption of ASU 2018-02.