Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 880,036	$ 963,296	$ 741,079
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	463,757	433,480	366,566
Impairment of assets	170,128	0	0
Allowance for equity funds used during construction (equity AFUDC)	(14,889)	(28,785)	(87,111)
Regulatory credit resulting from tax rate changes	(30,752)	(24,202)	(20,867)
Equity in earnings of unconsolidated affiliates	0	(3,216)	(1,068)
Distributions from unconsolidated affiliates	0	4,908	3,250
Changes in operating assets and liabilities:
Receivables - affiliates	(342)	582	91
Receivables - trade and other	14,742	(52,307)	(30,776)
Transportation and exchange gas receivable	1,733	(1,845)	(1,310)
Regulatory assets - current	6,579	26,330	1,379
Regulatory assets - non-current	11,361	3,622	(8,605)
Inventories	8,695	(1,787)	(23,178)
Payables - affiliates	(15,064)	(2,981)	7,307
Payables - trade	(915)	(18,842)	(35,869)
Accrued liabilities	22,881	(1,723)	40,993
Asset retirement obligations	12,027	15,713	24,659
Reserve for rate refunds	(188,842)	188,842	0
Deferred Revenue	(10,568)	(10,566)	(10,565)
Other, net	(36,990)	(36,365)	10,317
Net cash provided by operating activities	1,293,577	1,454,154	976,292
Cash flows from financing activities:
Proceeds from long-term debt	1,195,629	0	993,440
Proceeds from other financing obligations	8,725	41,513	50,269
Retirement of long-term debt	0	0	(250,000)
Payments on other financing obligations	(20,366)	(17,436)	(3,705)
Payments of debt issuance costs	(11,441)	0	(10,148)
Cash distributions to parent	(1,020,000)	(824,000)	(490,000)
Cash contributions from parent	115,000	0	340,000
Advances from affiliate, net	(252,549)	252,549	0
Net cash provided by (used in) financing activities	14,998	(547,374)	629,856
Cash flows from investing activities:
Capital expenditures	(649,459)	(901,374)	(2,326,672)
Contributions and advances for construction costs	35,284	47,034	408,912
Disposal of property, plant and equipment, net	(43,900)	(45,720)	(26,469)
Advances to affiliate, net	(642,734)	33,034	362,213
Contributions to unconsolidated affiliates	0	12,250	0
Purchase of ARO Trust investments	(53,357)	(72,946)	(51,793)
Proceeds from sale of ARO Trust investments	45,591	45,479	27,661
Other, net	0	37	0
Net cash used in investing activities	(1,308,575)	(906,780)	(1,606,148)
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	(582,896)	(854,761)	(2,085,888)
Changes in related accounts payable and accrued liabilities	(66,563)	(46,613)	(240,784)
Capital expenditures	(649,459)	(901,374)	(2,326,672)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	299,470	268,559	168,418
Income taxes	$ 352	$ 76	$ 632