Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 741,079	$ 55,728	$ 523,375
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	366,566	318,058	307,707
Allowance for equity funds used during construction (equity AFUDC)	(87,111)	(69,653)	(56,468)
Regulatory charge (credit) resulting from Tax Reform (Note 1)	(20,867)	471,096	0
Equity in (earnings) loss of unconsolidated affiliates	(1,068)	6,188	(5,914)
Distributions from unconsolidated affiliates	3,250	8,036	8,631
Changes in operating assets and liabilities:
Receivables - affiliates	91	(620)	595
Receivables - trade and other	(30,776)	(26,107)	5,941
Transportation and exchange gas receivable	(1,310)	(1,378)	600
Regulatory assets - current	1,379	(10,090)	(7,484)
Regulatory assets - non-current	(8,605)	(10,761)	(271)
Inventories	(23,178)	15,182	1,632
Payables - affiliates	7,307	12,514	(10,909)
Payables - trade	(35,869)	(9,823)	29,375
Accrued liabilities	29,740	(29,651)	74,759
Asset retirement obligations - non-current	34,075	103,105	31,114
Asset retirement obligation - removal costs	(9,416)	(4,578)	(4,911)
Deferred Revenue	(10,565)	(4,542)	0
Other, net	21,570	4,117	32,080
Net cash provided by operating activities	976,292	826,821	929,852
Cash flows from financing activities:
Proceeds from long-term debt	993,440	0	998,250
Proceeds from other financing obligations	50,269	0	0
Retirement of long-term debt	(250,000)	0	(200,000)
Payments on other financing obligations	(3,705)	(486)	0
Payments of debt issuance costs	(10,148)	(13)	(8,381)
Cash distributions to parent	(490,000)	(430,000)	(440,000)
Cash contributions from parent	340,000	410,000	502,000
Net cash provided by (used in) financing activities	629,856	(20,499)	851,869
Cash flows from investing activities:
Property, plant and equipment additions, net of equity AFUDC	(2,326,672)	(1,576,611)	(1,213,969)
Contributions and advances for construction costs	408,912	425,397	216,447
Disposal of property, plant and equipment, net	(26,469)	(49,090)	(12,529)
Advances to affiliate, net	362,213	416,446	(747,085)
Purchase of ARO Trust investments	(51,793)	(57,099)	(70,901)
Proceeds from sale of ARO Trust investments	27,661	31,435	44,195
Proceeds from insurance	0	3,200	2,121
Net cash used in investing activities	(1,606,148)	(806,322)	(1,781,721)
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, net of equity AFUDC	(2,085,888)	(1,784,254)	(1,200,696)
Changes in related accounts payable and accrued liabilities	(240,784)	207,643	(13,273)
Property, plant and equipment additions, net of equity AFUDC	(2,326,672)	(1,576,611)	(1,213,969)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	168,418	136,439	103,391
Income taxes	$ 632	$ 2,089	$ 828