Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 55,728	$ 523,375	$ 575,457
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	318,058	307,707	277,850
Allowance for equity funds used during construction (equity AFUDC)	(69,653)	(56,468)	(48,435)
Regulatory charge resulting from Tax Reform (Note 1)	471,096	0	0
Changes in operating assets and liabilities:
Receivables - affiliates	(620)	595	(430)
Receivables - trade and other	(26,107)	5,941	(19,521)
Transportation and exchange gas receivable	(1,378)	600	1,058
Regulatory assets - current	(10,090)	(7,484)	(1,765)
Regulatory assets - non-current	(10,761)	(271)	(24,650)
Inventories	15,182	1,632	9,858
Payables - affiliates	12,514	(10,909)	2,676
Payables - trade	(9,823)	29,375	(2,077)
Accrued liabilities	(29,651)	74,759	(10,015)
Asset retirement obligations - non-current	103,105	31,114	19,022
Asset retirement obligation - removal costs	(4,578)	(4,911)	(3,097)
Deferred Revenue	(4,542)	0	0
Other, net	14,415	32,030	45,007
Net cash provided by operating activities	822,895	927,085	820,938
Cash flows from financing activities:
Proceeds from long-term debt	0	998,250	0
Retirement of long-term debt	0	200,000	0
Payments on other financing obligations	(486)	0	0
Payments of debt issuance costs	13	8,381	0
Cash distributions to parent	(430,000)	(440,000)	(536,000)
Cash contributions from parent	410,000	502,000	652,000
Net cash provided by (used in) financing activities	(20,499)	851,869	116,000
Cash flows from investing activities:
Property, plant and equipment additions, net of equity AFUDC	(1,576,611)	(1,213,969)	(1,270,860)
Contributions and Advances for Construction Costs	425,397	216,447	85,901
Disposal of property, plant and equipment, net	(49,090)	(12,529)	(12,358)
Advances to affiliate, net	416,446	(747,085)	242,302
Return of capital from unconsolidated affiliates	3,926	2,767	2,015
Purchase of ARO Trust investments	(57,099)	(70,901)	(64,087)
Proceeds from sale of ARO Trust investments	31,435	44,195	43,284
Proceeds from insurance	3,200	2,121	35,132
Other, net	0	0	1,560
Net cash used in investing activities	(802,396)	(1,778,954)	(937,111)
Increase (decrease) in cash	0	0	(173)
Cash at beginning of period	0	0	173
Cash at end of period	0	0	0
Increase to property, plant and equipment, net of equity AFUDC	(1,784,254)	(1,200,696)	(1,222,292)
Changes in related accounts payable and accrued liabilities	207,643	(13,273)	(48,568)
Property, plant and equipment additions, net of equity AFUDC	(1,576,611)	(1,213,969)	(1,270,860)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	136,439	103,391	66,489
Income taxes	$ 2,089	$ 828	$ 1,161