Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019		Jan. 01, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	$ 10,958,229	$ 18,414,613
Losses reclassified from accumulated OCI, net of tax	118	(157)
Change in accounting principle			$ 0	[1]	$ 4,113
Ending Balance	11,139,588	16,863,762
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	0	4,625
Losses reclassified from accumulated OCI, net of tax	0	(287)
Change in accounting principle			0
Ending Balance	0	4,338
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(387)	(555)
Losses reclassified from accumulated OCI, net of tax	42	44
Change in accounting principle			0
Ending Balance	(345)	(511)
Other post- retirement benefit liability adjustment, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(5,019)	(6,528)
Losses reclassified from accumulated OCI, net of tax	76	86
Change in accounting principle			(496)
Ending Balance	(5,439)	(6,442)
Accumulated OCI, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(5,406)	(2,458)
Change in accounting principle			$ (496)
Ending Balance	$ (5,784)	$ (2,615)

[1] Related to adoption of ASU 2018-02. See Note 1 for additional information.