Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 523,375	$ 575,457	$ 422,884
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	307,707	277,850	269,395
Allowance for equity funds used during construction (equity AFUDC)	(56,468)	(48,435)	(18,701)
Changes in operating assets and liabilities:
Receivables - affiliates	595	(430)	1,947
Receivables - trade and other	5,941	(19,521)	17,437
Transportation and exchange gas receivable	600	1,058	3,272
Regulatory assets - current	(7,484)	(1,765)	(40,290)
Regulatory assets - non-current	(271)	(24,650)	17,532
Inventories	1,632	9,858	(19,167)
Payables - affiliates	(10,909)	2,676	9,420
Payables - trade	29,375	(2,077)	32,618
Accrued liabilities	74,759	(10,015)	(39,450)
Asset retirement obligations - non-current	31,114	19,022	30,840
Asset retirement obligation - removal costs	(4,911)	(3,097)	(12,824)
Reserve for rate refunds	0	0	(98,217)
Other, net	32,030	45,007	7,954
Net cash provided by operating activities	927,085	820,938	584,650
Cash flows from financing activities:
Proceeds from long-term debt	998,250	0	0
Retirement of long-term debt	200,000	0	0
Payments of debt issuance costs	8,381	0	0
Cash distributions to parent	(440,000)	(536,000)	(411,000)
Cash contributions from parent	502,000	652,000	267,000
Other, net	0	0	22,329
Net cash provided by (used in) financing activities	851,869	116,000	(121,671)
Cash flows from investing activities:
Property, plant and equipment additions, net of equity AFUDC	(1,213,969)	(1,270,860)	(724,163)
Contributions and Advances for Construction Costs	216,447	85,901	57,817
Disposal of property, plant and equipment, net	(12,529)	(12,358)	(7,532)
Advances to affiliate, net	(747,085)	242,302	219,470
Return of capital from unconsolidated affiliates	2,767	2,015	2,333
Purchase of ARO Trust investments	(70,901)	(64,087)	(52,038)
Proceeds from sale of ARO Trust investments	44,195	43,284	38,691
Proceeds from insurance	2,121	35,132	0
Other, net	0	1,560	2,503
Net cash used in investing activities	(1,778,954)	(937,111)	(462,919)
Increase (decrease) in cash	0	(173)	60
Cash at beginning of period	0	173	113
Cash at end of period	0	0	173
Increase to property, plant and equipment, net of equity AFUDC	(1,200,696)	(1,222,292)	(807,232)
Changes in related accounts payable and accrued liabilities	(13,273)	(48,568)	83,069
Property, plant and equipment additions, net of equity AFUDC	(1,213,969)	(1,270,860)	(724,163)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	103,391	66,489	77,304
Income taxes	$ 828	$ 1,161	$ 864