Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 575,457	$ 422,884	$ 374,026
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	277,850	269,395	263,949
Allowance for equity funds used during construction (equity AFUDC)	(48,435)	(18,701)	(13,299)
Changes in operating assets and liabilities:
Receivables - affiliates	(430)	1,947	55
Receivables - trade and other	(19,521)	17,437	(21,772)
Transportation and exchange gas receivable	1,058	3,272	(3,881)
Regulatory assets - current	(1,765)	(40,290)	28,536
Regulatory assets - non-current	(24,650)	17,532	21,910
Inventories	9,858	(19,167)	232
Payables - affiliates	2,676	9,420	(3,738)
Payables - trade	(2,077)	32,618	(26,463)
Accrued liabilities	(10,015)	(39,450)	28,322
Asset retirement obligations - non-current	19,022	30,840	13,105
Asset retirement obligation - removal costs	(3,097)	(12,824)	(26,919)
Reserve for rate refunds	0	(98,217)	98,217
Other, net	45,007	7,954	10,731
Net cash provided by operating activities	820,938	584,650	743,011
Cash flows from financing activities:
Cash distributions to parent	(536,000)	(411,000)	(250,000)
Cash contributions from parent	652,000	267,000	264,000
Other, net	0	22,329	(3,034)
Net cash provided by (used in) financing activities	116,000	(121,671)	10,966
Cash flows from investing activities:
Property, plant and equipment additions, net of equity AFUDC	(1,270,860)	(724,163)	(557,366)
Contributions and Advances for Construction Costs	85,901	57,817	30,450
Disposal of property, plant and equipment, net	(12,358)	(7,532)	(3,621)
Advances to affiliate, net	242,302	219,470	(214,215)
Return of capital from unconsolidated affiliates	2,015	2,333	1,438
Purchase of ARO Trust investments	(64,087)	(52,038)	(58,242)
Proceeds from sale of ARO Trust investments	43,284	38,691	45,607
Proceeds from insurance	35,132	0	0
Other, net	1,560	2,503	1,900
Net cash used in investing activities	(937,111)	(462,919)	(754,049)
Increase (decrease) in cash	(173)	60	(72)
Cash at beginning of period	173	113	185
Cash at end of period	0	173	113
Increase to property, plant and equipment, net of equity AFUDC	(1,222,292)	(807,232)	(572,956)
Changes in related accounts payable and accrued liabilities	(48,568)	83,069	15,590
Property, plant and equipment additions, net of equity AFUDC	(1,270,860)	(724,163)	(557,366)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	66,489	77,304	76,803
Income taxes	$ 1,161	$ 864	$ 116