Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 963,296	$ 741,079	$ 55,728
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	433,480	366,566	318,058
Allowance for equity funds used during construction (equity AFUDC)	(28,785)	(87,111)	(69,653)
Regulatory charge (credit) resulting from Tax Reform	(24,202)	(20,867)	471,096
Equity in (earnings) loss of unconsolidated affiliates	(3,216)	(1,068)	6,188
Distributions from unconsolidated affiliates	4,908	3,250	8,036
Changes in operating assets and liabilities:
Receivables - affiliates	582	91	(620)
Receivables - trade and other	(52,307)	(30,776)	(26,107)
Transportation and exchange gas receivable	(1,845)	(1,310)	(1,378)
Regulatory assets - current	26,330	1,379	(10,090)
Regulatory assets - non-current	3,622	(8,605)	(10,761)
Inventories	(1,787)	(23,178)	15,182
Payables - affiliates	(2,981)	7,307	12,514
Payables - trade	(18,842)	(35,869)	(9,823)
Accrued liabilities	(1,723)	40,993	(36,881)
Asset retirement obligations	15,713	24,659	98,527
Reserve for rate refunds	188,842	0	0
Deferred Revenue	(10,566)	(10,565)	(4,542)
Other, net	(36,365)	10,317	11,347
Net cash provided by operating activities	1,454,154	976,292	826,821
Cash flows from financing activities:
Proceeds from long-term debt	0	993,440	0
Proceeds from other financing obligations	41,513	50,269	0
Retirement of long-term debt	0	(250,000)	0
Payments on other financing obligations	(17,436)	(3,705)	(486)
Payments of debt issuance costs	0	(10,148)	(13)
Cash distributions to parent	(824,000)	(490,000)	(430,000)
Cash contributions from parent	0	340,000	410,000
Advances from affiliate, net	252,549	0	0
Net cash provided by (used in) financing activities	(547,374)	629,856	(20,499)
Cash flows from investing activities:
Capital expenditures	(901,374)	(2,326,672)	(1,576,611)
Contributions and advances for construction costs	47,034	408,912	425,397
Disposal of property, plant and equipment, net	(45,720)	(26,469)	(49,090)
Advances to affiliate, net	33,034	362,213	416,446
Contributions to unconsolidated affiliates	12,250	0	0
Purchase of ARO Trust investments	(72,946)	(51,793)	(57,099)
Proceeds from sale of ARO Trust investments	45,479	27,661	31,435
Proceeds from insurance	0	0	3,200
Other, net	37	0	0
Net cash used in investing activities	(906,780)	(1,606,148)	(806,322)
Increase (decrease) in cash	0	0	0
Cash at beginning of period	0	0	0
Cash at end of period	0	0	0
Increase to property, plant and equipment, exclusive of equity AFUDC	(854,761)	(2,085,888)	(1,784,254)
Changes in related accounts payable and accrued liabilities	(46,613)	(240,784)	207,643
Capital expenditures	(901,374)	(2,326,672)	(1,576,611)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	268,559	168,418	136,439
Income taxes	$ 76	$ 632	$ 2,089